speed under power

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Because my boat (FIRST 30 E, DISPL. 3,8t LWL 7,8m) had poor performance under power, I recently decided to change the original 13 HP (@ 3.600 RPM) RENAULT engine / gearbox (unknown reduction ratio), with a 17 HP (@ 2.800 RPM) KUBOTA engine fited with a HURTH Type 35 gear box. (forward reduction ratio A=2.05). Propeller remained the same (a two blade 15 x 11 "Radici" folding prop).
Speeds accomplished with old engine configuration were:
max 6.0 knots @ 3.200 RPM (calm water)
5,0-5,5 knots cruising @ 2.800 RPM.
With new engine configuration speeds accomplished are:
max 6.0 knots @ 2.200 RPM (maximum RPM accomplished with no exhaust black
smoke)
5,7 knots @ 2.100 RPM.
5,2 knots @ 1.900 RPM.
4,7 knots @ 1.700 RPM.
4,0 knots @ 1.500 RPM.

I believe that the boat can run better than that under power. Any sugestion will be welcomed. .
Stavros
 
I think your max revs achieved are a bit low. These small kubotas max at about 3600 rpm.(according to a spec sheet I have for the 3.75 HE. --Is that your motor?) If this is the case it looks like you are over propped. On the yanmarhelp .com site they advise running at 85% of max power. I know that is a different brand but i think the principles of keeping the engine loaded still apply. In your case you are still quite a long way from that.
I am gathering resources to get one of these motors for my boat as i think they offer a great power to weight/cost ratio. It might pay to trawl the second hand outlets to find a propellor that suits before you commit to an expensive final choice.
Keep posting as I am very interested in the outcome.

Cheers.
 
Firstly- calibrate your instruments. Are you sure these are accurate figures?

Second - what is the maximum hull speed of the boat. I would have thought a 30 ft boat was close to its maximum at 6 knots. At this speed is the hull noticeably stern down? with a couple of standing waves astern of you, across the wake? If so you are close to max hull speed.

Third - your speeds don't look that bad to me for 17 HP. Prop may be a little undersized, have you tried calculating the optimum using the downloadable program at http://www.propworks.com/ ?
 
Vyv may well be right about hull speed, I was focused on the engine performance,
however you still arent getting enough revs which doesnt seem to tally with an undersized prop.
 
Speaking as someone who knows almost nothing about it: If you change your engine then you should really change the prop as well.

Just plugged your figures into a prop calc spreadsheet - that suggests that your prop is too big (its suggestion is 13x9 - which seems very small to me). A too large prop would explain why you can't get the engine to full revs. Why not talk to a prop expert and see what they recommend?

However, I would also say that 6kts with a folding prop on a boat your size doesn't sound too bad to me
 
My first thought was to ask whether your RPM counter reads correct and thought you might have moved the alternator from the old machine to the new one with a different pulley size.

But doing the calculation on your boat inversely, taking your existing prop size, and boat paramters, you would need 37HP to turn the prop at 1756 RPM and with your gearbox the engine at 3600. This would push your boat well above hull speed to 9 kts.

The best to expect from 17HP would be around 6.9 kts. Your current prop doesn't allow your engine to reach max RPM and max HP, at the 2200 RPM you are probably getting about 11HP and that gives you about the 6kts. Your prop pitch should be about 9 to get the max from the engine. Diameter doesn't matter so much and your 15" would be ok.

Check if they can re-pitch your prop, or maybe they can sell you new blades.

I'm not a prop expert, this is just what I get using classic prop formulae and it seems to confirm your observations. A good prop shop should be able to help you.
 
Top speed looks only very-very slightly on the low side.

If memory serves me correctly the formula for optimum displacement hull speed is "Square root of waterline length in feet multiplied by 1.35 equals optimum efficient hull speed in knots."

In other words for the dimensions given this translates into 5.06 x 1.35 = 6.8 knots.

I have a similar dimensioned boat with a 72 HP engine.

Speeds are:
o 1,200rpm - 4 knots
o 1,800rpm - 6 knots
o 2,600rpm - 8 knots
o 3,600rpm - 9 knots

At 1800rpm she burns about 5 litres of diesel an hour whereas at 3,600rpm she burns >15 litres per hour. A 300% + increase in fuel consumption for a 50% increase in speed dramatically demonstrates the "optimum hull speed" theory!!!

Hope this helps.

Best regards :o)

Ian D
 
Re: Probably wrong size prop

I have used two calculation programs and plugged in your data:

1. Hull speed - http://www.anyboat.com/hull.htm This confirms DoctorD's thought that hull speed is approaching 7 knots

2. PropCalc - http://www.users.globalnet.co.uk/~cmprops/ which gives me 14.3 x 12.5, not too far away from what you have. If the rpm you quoted is the maximum your engine pulls, you may have a problem with your rev counter.
 
Don't have the technical understanding displayed here, but I believe you should with your strong engine reach hull speed of 6.8kn.
And you apparently do not.
That seems to be in line with the better funded assessments of the others, anyway.
with a boat your work never stops!
chris
 
Re: Probably wrong size prop

Does anyone have a rule of thumb guide to how much more efficient a fixed 3 blade is over a folding 2 blader?
 
My experience is as follows, I have a Jaguar 27 with a Perkins Perama of 17HP. It would only rev under load to about 2500 RPM, and I decided the boat was overpropped(15 x 9 - 2 bladed fixed) and had the prop re-pitched to 15 x 8, and afterwards it revved to 2250 RPM, close to the maximum of 3600. On speed trials flat out I averaged 6.5 knots max. Good enough for me.
 
Re: Probably wrong size prop

Since I am convinced that something has to be done with the propeller (reduce pitch even though no black exhaust smoke is present at max RPM achieved under load (2200RPM) ), and I have the possibility to increase the no load engine max RPM from 2800 to 3200 RPM by means of a stop screw, (close to the max 3600RPM of the original engine), I plan do some testing and sea trials during the weekend. Will keep you informed of any drastic results
I want to thank all of you for your helpfull advise. I have gathered sufficient data and ideas to keep me busy the whole weekend.
Stavros.
 
If you ring T.W.Norris in Isleworth and speak to them they will advise you on the best prop size for your configuration.
 
A comparison (for what it\'s worth).

My Westerly Fulmar has Volvo 2002, allegedly 18hp but has about 5000 hours now so call it 17hp.

In crusing trim she weighs about 5.5 tonnes, 32ft LOA, 26ft LWL.

Flat water figures with clean bottom and batteries well charged (alternator can take a couple of HP I'm told)...

Hull speed 6.3 knots at 2,600 rpm
Max out at 3,400rpm gives 7.2 knots
 
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