New engine or old prop... or something else?

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seafox67

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New engine went in last year (Volvo D1-20) to replace the original 1988 Volvo 18HP. I did not replace the the shaft and prop though.

During my testing, I have found that after applying 40% of throttle the speed or revs do not increase very much. The boat max's out about 1 knot below max hull speed.

On 40% throttle, the engine runs at about 1800 RPM and on full throttle increases to around 2300 RPM but no more significant increase in speed. The boat is Westerly Tempest 31.

What do you think?
A. Normal
B. Engine issue
C. Prop issue
D. Something else
 
scottie said:
Was it brand new or just new to you?
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The engine was brand new.
I spoke to the installers at the time about the prop and although they said the prop was a little oversized, it was decided that it wasn't enough to make much difference.

I'm guessing from my inexperienced mind that the prop will need changing but it would be prudent to get a few opinions before going through that time and expense.
I would be gutted to change it and find no difference at all :(
 
Speak to prop suppliers
I know that it’s tempting to save money and use the old prop but I suspect that the gearbox ratio will be different
The old engine would produce 18 hp at approx 2500 rpm the new 20hp @ 3200?
I am a little surprised that the engine not making full revs under load was acceptable to the person signing off the installation as unless the system has changed since I retired Volvo should not accept the warranty registration with that sort of overloading!
 
scottie said:
Speak to prop suppliers
I know that it’s tempting to save money and use the old prop but I suspect that the gearbox ratio will be different
The old engine would produce 18 hp at approx 2500 rpm the new 20hp @ 3200?
I am a little surprised that the engine not making full revs under load was acceptable to the person signing off the installation as unless the system has changed since I retired Volvo should not accept the warranty registration with that sort of overloading!
Click to expand...

Good point... I was onboard when the engineer took the boat out for a sea test and he did write down the figures for the warranty. I'll have a look for them.

That does suggest something may have changed, other things were left from the old engine like the throttle and linkages.

The boat was last pulled out in September when the engine was replaced so maybe there is enough crap on the hull to make a difference. She's a bilge keel so beach her and clean might be a cheaper option to try first
 
I've just had a look at the D1-20 spec sheet. The max RPM range is 3200 however the torque max's at 2400 and then drops away.

2400 RPM is around all I can get. Do you think this is a coincidence?
 
Very much sounds as though you're over-propped.
Presumably the new engine came with a new gearbox? If so, is the drive ratio the same as before? Until that's clarified, any other considerations are moot.

The torque to rpm figures you mention are typical of small marine diesels.
 
macd said:
Very much sounds as though you're over-propped.
Presumably the new engine came with a new gearbox? If so, is the drive ratio the same as before? Until that's clarified, any other considerations are moot.

The torque to rpm figures you mention are typical of small marine diesels.
Click to expand...

Yep... I got a new gearbox.
The old gearbox was MS2B 852716 3.02
and the new gearbox is MS16A B 2.63
 
seafox67 said:
Yep... I got a new gearbox.
The old gearbox was MS2B 852716 3.02
and the new gearbox is MS16A B 2.63
Click to expand...

Seems you are overpropped. The sign off for sea trials on a new installation with a D1 is 3000-3100, so if you are only getting 2400 that suggests you need to take off up to 2" of pitch. Really surprised whoever supplied the engine did not do the calculations for prop size as maximum revs and reduction ratio (hence shaft speed) are quite different.

You might try putting your boat/engine details into the Propcalc programme on www.castlemarine.co.uk to get a ball park size. Alternatively ask a supplier such as T Norris or Lake Engineering for a recommendation.
 
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macd said:
There's your answer, or a major part of it.
The installers really should have considered that. Installation 101.
Click to expand...

Thanks, I do remember discussing the prop at the time of installation but I got the impression it would be marginal. However, this does not feel marginal and I'm guessing it is causing some damage to my new engine!
 
Do I understand that you have not used the yacht since September (this is an alien concept in Australia where we can sail 365). Surely once you had the new engine you enjoyed using it - I would have been racing around trying it (and the bigger alternator powering all the gizmos) - so how was it in, say October. The difference, or the poor performance, is not something you would miss - so why raise the issue now, not then.

A fouled hull, or poly bag round the prop - would be a quick and easy place to start (and get those commissioning figures). Also to enjoy warranty you should have the first service done by a Volvo dealer (I think that's the deal) - now seems a good time!

And it should be under warranty anyway - and should have met specs then.

Jonathan
 
What pitch and diameter is your prop. It should have the numbers stamped on it eg 12 * 14. One number is the diameter and the other is the pitch ,i.e. how far forward a single turn of the prop will theoretically make the prop move.
If you know these numbers I'm sure there's at least 1 or 2 other owners on the forum who could tell you how those numbers compare to thier boat and the gear box ratios . I don't have the gear box ratio to hand ( guess it's 2.5) but we are over propped on our Fulmar with a 16" pitch, Please note we chose to do this and get to max speed at 2800 revs of the 3500 avaliable.
 
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By load restricting the max. revs to ~2300, your engine can only produce ~15 hp, as against the 20 hp or so it is capable of . Hence your poor max. speed.
Max. torque revs. is irrelevant.
The ratio difference of the gearboxes is quite significant, as said prop. is too big.
Can't see the engine being damaged unless you spend lengthy periods at WOT, but I'd re-size the prop.
 
You can also get your current prop re pitched, however not sure of the cost and you may decide that you fancy a lovely folding one like we have from Darglow, or possibly a variable pitch prop.
 
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