10, 14 or 21hp?

[Tranona]

In my opinion, I would base my decision on frequency and sea conditions. How often I will be using the engine for motor sailing and how exposed the local sea area is. In the main, I would prefer a larger engine (21 hp) so that I can operate it at reduced rpm to obtain the optimum torque, (presumably that the 14 hp engine would be developing its peak torque at a higher rpm than the 21 hp); the extra hp would be useful for when going against tide and wind.

Not a good idea with these small engines. They are designed to run at 70% maximum power continuous, so putting in a larger engine where you cannot absorb the power is pointless. Torque is largely irrelevant as you will see that they have very flat torque curves over the operating range from 1600 - 3200 rpm.

As noted earlier this boat does not have the space to swing a 14" prop which is the minimum size for a 21hp. A 14hp with a 13" prop will easily achieve maximum hull speed and cruise at 5 knots at around 2600 rpm which is about 65% max power. Engine manufacturers will not sign off installations that do not achieve close to maximum revs, which in this case will be 3400-3600 rpm.

 

[JumbleDuck]

As noted earlier this boat does not have the space to swing a 14" prop which is the minimum size for a 21hp.

A four-blade Featherstream ought to cope.

 

[charles_reed]

A four-blade Featherstream ought to cope.

I think you'll find that prop diameter is far more important than the number of blades when selecting the propeller for a given hull/engine combination.

 

[JumbleDuck]

I think you'll find that prop diameter is far more important than the number of blades when selecting the propeller for a given hull/engine combination.

Diameter certainly matters a lot, but there is no doubt that a four-blader soaks up more power than an otherwise identical three-blader. That's why four-blade props are normally pitched an inch less than three-blade ones for the same application.

 

[LittleSister]

I've been out in the boat today (mainly a cracking sail, but I did have the engine (Beta/Kubota 14 twin) on for an hour or so), and deliberately tried to concentrate on the vibration because of your concern, but kept quickly forgetting because it really is not any sort of nuisance. I can feel vibration with my feet on the cockpit floor if I think about it, but can't remember ever being conscious of it before.

I've just asked the First Mate, who is very fussy, and easily disturbed by all manner of things, and she says she's never been bothered by vibration on the boat (but hates the noise of it, and any smell of diesel).

Obviously vibration will vary between different boats and installations, but I really think that unless you are hyper-sensitive to vibration it is not really worth spending much (if any) extra money to get something smoother than the Kubota twin.

 

[LittleSister]

presumably that the 14 hp engine would be developing its peak torque at a higher rpm than the 21 hp

From looking at the specs it seems both the 14hp and 21hp develop their maximum torque at 2,600 rpm.

 

[Tranona]

Diameter certainly matters a lot, but there is no doubt that a four-blader soaks up more power than an otherwise identical three-blader. That's why four-blade props are normally pitched an inch less than three-blade ones for the same application.

Yes, but you still will not be able to use all the power the engine is capable of producing, so you will just get to your 6 knots drawing 2/3 of the hp and squatting into your stern wave!

 

[LittleSister]

I believe it is 360 degrees

I believe that the Kubota twin engine used by Beta for their 10 and 14 is 180 deg. ie uneven firing. Presumably the Nanni is the same.

I stand corrected. Having just found a photo of a Kubota twin crankshaft it looks like 180 degrees, and definitely not 360.

 

[JumbleDuck]

Obviously vibration will vary between different boats and installations, but I really think that unless you are hyper-sensitive to vibration it is not really worth spending much (if any) extra money to get something smoother than the Kubota twin.

Thanks. The 1GM10 really only vibrates significantly at tickover, so it's not a huge concern to me, but it's nice to hear your experience of the twin.

 

[Pye_End]

Yes, but you still will not be able to use all the power the engine is capable of producing, so you will just get to your 6 knots drawing 2/3 of the hp and squatting into your stern wave!

There was a Victoria 26 around with a Beta 21 which did exactly that - and complained of wet feet for those standing in the cockpit. Not sure which prop was used.

Think it was this one:


BTW Ian - if you are still on your original mild steel diesel tank, then it is a good opportunity to change it.

 

[nathanglasgow]

Had the 1gm10 in our last boat,a Halcyon 27. Found in underpowered and by god did it bounce around at idle but all the same great little lump,easy to work on and reliable. Current boat has a Beta 14( Dufour Arpege) and is probably 4000kg in cruising trim. Massive difference in smoothness and will push along at 6 knots in calm waters at 2800 ish but that always feels like it's revving the backside off it. 2200 feels much more relaxed and gets 5 knots,however in a lumpy sea it's down to 2 ish know driving into it and it's then we wish for more horses. Fit the biggest you can. Re the heat exchanger, never had any problems,just check anode every couple of months and change it if it looks wasted. Heat exchanger gets removed and cleaned every 2 years but could probably extend that to 3 years as never very manky.

 

[nathanglasgow]

Oops forgot pic of heat exchanger after two years, quite clean.

 

[JumbleDuck]

There was a Victoria 26 around with a Beta 21 which did exactly that - and complained of wet feet for those standing in the cockpit. Not sure which prop was used.

Mine squats enough to get water in the cockpit with a 1GM10. Mind you, I am a larger gentleman.

BTW Ian - if you are still on your original mild steel diesel tank, then it is a good opportunity to change it.

My original tank is stainless, thanks to the purchaser ticking the options list sensibly. However, I am think of replacing it anyway, and putting a wedge-shaped plastic tank at the bottom of the cockpit locker.

 

[CAPTAIN FANTASTIC]

Yes, but you still will not be able to use all the power the engine is capable of producing, so you will just get to your 6 knots drawing 2/3 of the hp and squatting into your stern wave!

There is no problem reaching full hull speed with a small engine in ideal conditions. The extra horse power is needed for when going against tide and rough conditions; this is why when choosing an engine the decision should based on a number of issues including sea conditions expected to be used.

 

[JumbleDuck]

There is no problem reaching full hull speed with a small engine in ideal conditions. The extra horse power is needed for when going against tide and rough conditions; this is why when choosing an engine the decision should based on a number of issues including sea conditions expected to be used.

Exactly. At the moment I can just reach 6kt with whatever an elderly 1GM10 produces ... but only on a flat sea in a calm. By 20kts of wind I am down to 4kt ... add a few waves and progress is laboured. I'm quite happy with the 1GM10 as an auxiliary, but would like a bit more oomph.

 

[lw395]

There is no problem reaching full hull speed with a small engine in ideal conditions. The extra horse power is needed for when going against tide and rough conditions; this is why when choosing an engine the decision should based on a number of issues including sea conditions expected to be used.

Hull speed is through the water.
Ironically you often need a lot more power going with the tide, if it's against the wind....

 

[lw395]

I've been out in the boat today (mainly a cracking sail, but I did have the engine (Beta/Kubota 14 twin) on for an hour or so), and deliberately tried to concentrate on the vibration because of your concern, but kept quickly forgetting because it really is not any sort of nuisance. I can feel vibration with my feet on the cockpit floor if I think about it, but can't remember ever being conscious of it before.

I've just asked the First Mate, who is very fussy, and easily disturbed by all manner of things, and she says she's never been bothered by vibration on the boat (but hates the noise of it, and any smell of diesel).

Obviously vibration will vary between different boats and installations, but I really think that unless you are hyper-sensitive to vibration it is not really worth spending much (if any) extra money to get something smoother than the Kubota twin.

The vibration is what causes a lot of the noise.
My 1GM used to make a lot of noise vibrating the structure of the boat.
To some extent this was probably due to it being a retrofit on an Impala originally designed with an outboard in a well.

Didn't worry me as I tend to use the sails as much as possible, or stay where I am in preference to motoring all day.

 

[Pye_End]

There is no problem reaching full hull speed with a small engine in ideal conditions. The extra horse power is needed for when going against tide and rough conditions; this is why when choosing an engine the decision should based on a number of issues including sea conditions expected to be used.

Back to the prop - only if you can transmit the power. Gearbox ratios are the same, as are engine rpm's. You can get near full rpm with a 13 x 8 with the 14.

Perhaps the 16 might allow a slightly deeper pitch? Could go 13.5 diameter if you don't mind being so close to the hull.

 

[Tranona]

There is no problem reaching full hull speed with a small engine in ideal conditions. The extra horse power is needed for when going against tide and rough conditions; this is why when choosing an engine the decision should based on a number of issues including sea conditions expected to be used.

While that is true to an extent, overpowering a boat is wasted. on the boat in question, maximum hull speed in flat water can be achieved with little more than 10hp. Doubling the amount of power will not enable any better performance - even in adverse conditions.

As I said, manufacturers design engine/box/prop combinations to achieve maximum hull speed at close to maximum revs. You simply cannot use the excess hp on an overpowered engine, so have to run it at less than optimal revs.

Extra power does not help going against tide, only against wind and waves. Even then there is a limit as it is the propeller than moves the boat, not the engine. so if the propeller cannot demand the hp, the bigger engine is wasted.

 

[Bob Stay]

It's worth noting that the so-called 'hull speed' of a 'double ender' is slower than that of a transom hulled boat of the same LWL. ...

Really? Many yachts are double enders below the waterline. Got any evidence for your assertion, because it sounds like an old wives' tale.
My double ender is extraordinarily fast, under sail or power easily able to exceed in knots the trad formula: 1.3 x sq root LWL in feet.
How, in your opinion, should the formula be modified to apply to a double ender?