Too big an engine or too small a prop?

[chrisbitz]

My new boat 26ft Salty Dog (yaay!) was reengined a few years ago, and the guy chose to put a 20hp Penta in to replace the standard 10hp.

I've never had an inboard before, but to me, it feels like the engine is never under much load, and needs to rev quite a lot to get any sort of speed up like 4-5 kn.

Is this normal, or my theory is that he's left the original saildrive prop on there, with twice the HP driving it?

It's a folding prop too, and people have said that a new prop is around £400?

Any comments or suggestions?

 

[Coaster]

If using the engine for much of the time, and particularly if you make a habit of motorsailing, a Bruntons Autoprop would improve your installation considerably. It's self pitching so acts rather like an automatic gearbox. Under quiet conditions 'hull speed' is achieved at low revs, unlike a fixed pitch prop (whether folding or not).

We've been very pleased with ours although other people may have different opinions.

When dealing direct with Bruntons in 2009 we obtained a roughly 30% discount on list price. I don't know what's available now.

 

[earlybird]

You need to look up the rpm for maximum power output of your engine. Probably around 3400 to 3600 rpm. It will be quoted in the manufacturer's spec.
The engine and propeller should be matched so that at full throttle, preferably in fairly smooth water, the engine reaches that rpm, or perhaps at least 90%. If it reaches full rpm at much less than full throttle, the prop is too small and the engine revs will be unnecessariy high for a given boat speed.
Obviously, a tacho is needed for this, if one is not fitted, ebay do cheapos, although I've not tried one.
BTW; you won't get a new folding prop for £400.

 

[Tranona]

The answer to your basic question is that, yes a prop for a 10hp driven saildrive will be too small for a 20hp. It is relatively easy to estimate the size of prop by putting your boat's data into Propcalc on www.castlemarine.co.uk However, a problem you may have is that saildrives have different reduction ratios depending on the type and if the new engine is hooked up to an old saildrive there may be a mismatch from standard. So you need to know the exact type of engine you have and the exact model of saildrive. The prop size for a 20hp is normally 15" diameter, but the pitch will vary according to the reduction ratio and to an extent the type of prop. A new standard fixed 2 blade Volvo prop is just under £300, and a folding prop well over twice that. However you may be able to get new blades for the folding prop to change both diameter and pitch, depending on the make. Find out what make of prop it is and ask the distributor for a recommendation for your set up. A specialist like Sillette Sonic may also be able to help you.

 

[KellysEye]

When fitting a new larger engine it is normal to replace the prop shaft and prop, that was done when our old Volvo MD17C 35HP was replaced by a 52HP Volvo engine. My guess from what you have said is that wasn't done, you could ask the previous owner or the mechanic that fitted the engine.

 

[Topcat47]

Can you fit a Brunson Autoprop to a Saildrive?

You will also need to know the pitch and diameter of your current prop, the reduction ratio of the saildrive and the RPM at peak power and torque before deciding if it's the prop.

It's quite common to fit larger engines than the hull warrants. I was "advised" to fit a 28hp to my boat so your 20hp isn't extreme (I stuck with a 10 and it's OK in 90% of conditions). You won't use "full power" to drive the boat under most conditions. The Saltydog ought to make 5-6 knots before cavitation sets in. If this is at the top of the engine's speed range, that's all you'll get.


It sounds to me as if your prop's pitch is a bit fine for the engine, which I'd find surprising if it had been an old prop sized for an older 10hp engine.

 

[Yara]

There is no problem with what you have if the revs suit the engine. See what power and revs is required to produce hull speed in calm water. Check with the manual, and if the number is say 50 or 60% of max rpm, then you have plenty left to use when you have to battle a headwind and rough sea.

 

[ianj99]

My new boat 26ft Salty Dog (yaay!) was reengined a few years ago, and the guy chose to put a 20hp Penta in to replace the standard 10hp.

I've never had an inboard before, but to me, it feels like the engine is never under much load, and needs to rev quite a lot to get any sort of speed up like 4-5 kn.

Is this normal, or my theory is that he's left the original saildrive prop on there, with twice the HP driving it?

It's a folding prop too, and people have said that a new prop is around £400?

Any comments or suggestions?

As Tranona suggested, use propcalc to get an idea of the new size, but enter the rpm for 10hp, not 20.
(Look up the power curve for the 20bhp engine and find the rpm for 10hp)

This will give a prop sized to give the same performance* as the original engine but at much lower, quieter and more fuel effecient rpm, yet still give a reserve for when conditions demand, something that wasn't available with the 10bhp engine.

(*I've assumed that 10bhp was enough to achieve hull speed in ideal conditions. Hence there is no need to match the prop to the 20bhp rpm when you don't need it.

There is a Kiwi feathering prop available for sail drives and they are much cheaper than others.

http://www.vectamarine.com/kiwi-prop.html

 

[Tranona]

As Tranona suggested, use propcalc to get an idea of the new size, but enter the rpm for 10hp, not 20.
(Look up the power curve for the 20bhp engine and find the rpm for 10hp)

This will give a prop sized to give the same performance* as the original engine but at much lower, quieter and more fuel effecient rpm, yet still give a reserve for when conditions demand, something that wasn't available with the 10bhp engine.

(*I've assumed that 10bhp was enough to achieve hull speed in ideal conditions. Hence there is no need to match the prop to the 20bhp rpm when you don't need it.

There is a Kiwi feathering prop available for sail drives and they are much cheaper than others.

http://www.vectamarine.com/kiwi-prop.html

That will not give you the answer. The prop needs to allow the engine to run at max revs. 20hp is not too much for a 26' boat - 10hp was not enough, but the boat is constrained by having a saildrive so the options are limited to what is available in the Volvo range if you want to re-use the saildrive as it seems has been done. The OP already has a folding prop and a replacement will be cheaper than a Kiwi, and even cheaper if he can just buy new blades.

 

[Tranona]

When fitting a new larger engine it is normal to replace the prop shaft and prop, that was done when our old Volvo MD17C 35HP was replaced by a 52HP Volvo engine. My guess from what you have said is that wasn't done, you could ask the previous owner or the mechanic that fitted the engine.

Please read the question. The boat has a saildrive so all what you say is irrelevant!

 

[ianj99]

That will not give you the answer. The prop needs to allow the engine to run at max revs. 20hp is not too much for a 26' boat - 10hp was not enough, but the boat is constrained by having a saildrive so the options are limited to what is available in the Volvo range if you want to re-use the saildrive as it seems has been done. The OP already has a folding prop and a replacement will be cheaper than a Kiwi, and even cheaper if he can just buy new blades.

If the OP follows this advice, he'll be in exactly the position he is complaining about now.
Its not about whether 20bhp is too much, its about how much he needs to achieve a) hull speed, and b) a quiet economical crusing speed.

In my own case, my 4018's max rpm is 2400rpm at 7kts (~ hull speed), crusing speed is 6kts at a quiet and smooth 2000rpm. By setting the Kiwi's pitch 20% higher than it was when a fixed prop was fitted, I sacrifce a few bhp by having the rpm restricted to 2400 instead of 3000, but in adverse conditions when those few bhp could be useful, I am happy to just go a bit slower.

The old school of thought is to match prop for max power and rpm - fine if you want or need all that the engine can produce otherwise, if the engine is larger than you need for 95% of the time, over prop it for a more relaxing crusing speed.*

I think this old rule of thumb is due for the bin - it stems from the days when marine engines were generally lower powered than these days, and also the smallest engine was fitted (probably on grounds of cost) so there was no reserve to allow over propping.

* the same argument applies to motor vehicles which now have a very high top gear - one which won't allow the engine to reach max bhp rpm, in the interests of relaxed crusing and fuel economy.
Ian

 

[Tranona]

Suggest you ignore your own situation. The engine is entirely suitable for the boat. It is not achieving boat speed almost certainly because the prop is too small. Fitted with the correct prop it will achieve satisfactory cruising and maximum speed using the full range.

BTW it is not "old school of thought" to choose a prop that maximises rated rpm. It is the advice of all manufacturers. The Volvo saildrive combination in question is intended to run up to within approx 200 rpm of maximum (which is 3600 if the engine is a 2020) with the standard prop provided with the engine. This will allow comfortably cruising at approx 70% or max revs, that is around 2400-2500. The only unknown here is whether the drive/engine combination is standard. If the drive was originally on a 10hp engine it would have used a smaller prop than if it is hooked up to a 20hp. That is why I suggested the OP determines exactly which drive and engine type he has as you can then look up the Volvo prop selection chart to choose the right prop if it is a standard setup, or make adjustment to the pitch if a non standard combination.

 

[grumpy_o_g]

That will not give you the answer. The prop needs to allow the engine to run at max revs. 20hp is not too much for a 26' boat - 10hp was not enough, but the boat is constrained by having a saildrive so the options are limited to what is available in the Volvo range if you want to re-use the saildrive as it seems has been done. The OP already has a folding prop and a replacement will be cheaper than a Kiwi, and even cheaper if he can just buy new blades.

10hp can be plenty for a 26' boat though I agree 20 hp is better in some circumstances. Did you mean to say that the OP is limited to what is available in the Volvo range given Kiwi, Brunton and others can be fitted to a Volvo saildrive. A new Brunton Autprop would probably cost about the same as a new engine though Can you re-pitch folding prop blades?

 

[Tranona]

10hp can be plenty for a 26' boat though I agree 20 hp is better in some circumstances. Did you mean to say that the OP is limited to what is available in the Volvo range given Kiwi, Brunton and others can be fitted to a Volvo saildrive. A new Brunton Autprop would probably cost about the same as a new engine though Can you re-pitch folding prop blades?

10hp is marginal these days - even in the past 26 footers were fitted with 23 hp engines (think Westerly Centaur!) Through choice a 15/16 hp engine would be OK, but until recently if you had a Volvo saildrive your choice was 10 or 20 - former marginal, latter a bit overkill. No, I did not mean that you are restricted to Volvo props, just that I guess on an old boat like that spending £2k on a Bruntons is perhaps not what the OP wants to do! The standard Volvo 2 blade aluminium prop is perfectly adequate for both sailing and motoring - and "cheap". However the OP has a folding prop, and with many of those you can buy new blades to change both diameter and pitch. My concern is that it is probably a 110 drive which is way obsolete and the prop may also be very old. That is why the OP has to find out exactly which drive he has and which engine before he can determine the options open to him. There are 3 different reduction ratios and 4 different types of engine, each with different power characteristics, so therefore different prop specifications!

 

[PeterMcK]

The engine needs to run at maximum rpm only if you need maximum power output. The OP needs less than that so he has the option of running at full rated torque but less than max rpm or, as now, maximum rpm but less than rated torque.

My engines have had a maker specified range for continuous use of 1000 rpm to 3000 rpm (and 3500 rpm for intermittent use). If Volvo say that their small engines are fit only to run at full speed, then there's something badly wrong with them; I don't think there is.

Tranona's advice is correct. Your boat probably need no more than 10-15 hp to get up to hull speed. "Gear" it appropriately. Either use an online calculator to assist with prop selection or speak to a specialist supplier.

Sillette Sonic supply Radice two blade folders for shafts and saildrives starting at £435 (and there is almost certainly a discount to be had). They might not be as pretty as Goris but they're robust, functional, reasonably efficient and a lot cheaper. If you already have a Radice, you can re-blade for 60% of the prop price (that's how I recently increased my own pitch and diameter after re-engining).

 

[Tranona]

Sillette Sonic supply Radice two blade folders for shafts and saildrives starting at £435 (and there is almost certainly a discount to be had).

Not sure where you get the prices from. A 15" Radice saildrive folding prop (which is the size the OP needs) is £630. A Fixed prop of the same size is £228.

 

[PeterMcK]

http://www.sillette.co.uk/price_pdf_files/sillette_marine_propulsion_catalogue.pdf

Someone said you can't get a folding prop for £400.

Page 9, they start at £435. And are probably less if you ask for a quote.

OP doesn't actually know what size he needs yet.

BTW, my Moody 33, in common with all standard configuration M33 Mks 1 and 2, and M33Ss, has no room to swing a big prop, so the 35hp Thornycroft T90 powers through a 1:1 gearbox and a 12" diameter propeller (13" x 10" pitch in my case with the non-standard 50hp T108 has the engine fully loaded at 2500 rpm and gives about 8 knots, well above hull speed). My Radice would, in fact, cost only £435, or about two thirds the price of a Gori which, in any case, has very limited pitch options and wouldn't have suited.

 

[Tranona]

http://www.sillette.co.uk/price_pdf_files/sillette_marine_propulsion_catalogue.pdf

Someone said you can't get a folding prop for £400.

Page 9, they start at £435. And are probably less if you ask for a quote.

OP doesn't actually know what size he needs yet.

Suggest you read the OPs requirements before posting misleading information. He has a saildrive, which for the engine now fitted to his boat, needs a 15" prop. And assuming he lives in the UK he will have to pay VAT (and carriage) so a folding prop to suit his boat will cost £630 and a fixed £228.

Pointless comparing with what you have on your boat as that is not suitable for his - and I assume you will have to also pay VAT bringing the retail price of your prop to £522.

So, it is not possible at retail prices to buy any kind of folding prop new for £400 - even a Radice, and folding props are typically at least twice the price of equivalent fixed props.

 

[PeterMcK]

Suggest you read the OPs requirements before posting misleading information. He has a saildrive, which for the engine now fitted to his boat, needs a 15" prop. And assuming he lives in the UK he will have to pay VAT (and carriage) so a folding prop to suit his boat will cost £630 and a fixed £228.

@ Tranona,

Read my words and take the literal meaning, not what you imagine me to mean.

And BTW, I read the OP's post - again read what I said. Is clear grammatical English confusing to you?

 

[Tranona]

@ Tranona,

Read my words and take the literal meaning, not what you imagine me to mean.

And BTW, I read the OP's post - again read what I said. Is clear grammatical English confusing to you?

No, just that what you are saying is not relevant to the OPs situation. He has a saildrive. You cannot buy a folding propeller for £400 - so why are you suggesting he can? Even if he has a shaft drive the cheapest is £522 including VAT - less any discount he might be able to get.

You should also read what I have suggested he do. Until it is known exactly which engine and saildrive he has you cannot determine the correct prop. There are 4 possible engines and 3 possible saildrives. If it originally had a 10 hp engine it would have a 13 or 14" prop. A 20 hp normally uses a 15" prop. The saildrive is the same for all engines up to 50hp, but different ratios depending on which engine they were fitted to. The props are all the same spline size so you can have from 13 to 17 or 19" (depending on which drive type), with 2 or 3 blades and a range of pitches in each so that you can match the prop to individual boats.

You will also see that I have already suggested he might be able to replace the blades and that he speaks to Sillette, but that is all conjecture until he knows what type and size of prop he has.

Volvo do not recommend you run your engine continuously at maximum revs. They recommend you fit a prop that enables you to achieve within 200rpm of maximum at WOT, which on a 2020 engine which the OP probably has is between 3400 and 3600. Cruising speed is then at around 2400 rpm. Same recommendation as all the other small diesel manufacturers such as Yanmar. It is shaft speed that is important for propulsion, so saildrives have different reduction ratios because engines have had different maximum RPM - the D1 20 is 3200 for example, but as standard most end up with a maximum shaft speed of 1500.

My boat weighs approx 6000kgs and does very well with 29 hp and a Saildrive with a 16*11 2 blade Flexofold. The design weight of your boat is under 5000kg but you have potentially 50hp, but you can't use it because your shaft speed is too high (nearly double the norm) and you cannot swing a big enough diameter prop to absorb the power. The OPs boat (like mine) does not have those kind of constraints so what you do to make yours work is not transferable to my boat or his.

So, apart from posting a link to a price list, not sure what you have done to help the OP.