Proper size for sailboat diesel ??

[RDKinard]

We've been told our Universal Diesel (Model #5432 - 32 hp) is undersized for our 37' O'Day sloop (30'6" LWL). During rally's we have a hard time keeping up with boats our size, averaging only 5-1/2 to 6 knots vs our theoretical hull speed of 7.5 knots. Our engine mechanic has optimized our maxi-prop pitch so the diesel runs at maximum allowed rpm, 2800 under load, 3000 unloaded.

Does anyone have a good way to calculate what is a properly sized diesel? (i.e. hull drag @ 90% theoretical hull speed & average operating current converted to hp)

Captain Rich

[SeaDrive]

Your best bet may be to contact the designers:
C. Raymond Hunt Associates, Inc.
69 Long Wharf
Boston, MA 02110-3618 USA
Tel: 617-742-5669
Fax: 617-742-6354
Email: info@crhunt.com

A typical rule of thumb is 4hp/ton. That would seem to suggest your 32hp is about right, but what do I know?

[FAST FRED]

My guess is that you don't Actually Cruise at 2800rpm , but some lower speed.

You will find more pitch may help , even if you cant pull full rpm.

This is whats called a cruising prop , not recomended by most Mfg.

The Mfg worries that you will have the oversized prop AND run at full bore.

To give the engine longest life record max RPM and then NEVER operate at more than 90% of that number.

FAST FRED

[Robert Miller]

32 HP is more than your boat can actually use. A larger engine will just spend your money needlessly.

Something else is afoot here.

My first thought, is that pitching the prop to attain maximum engine RPM is not the proper approach in any case.

IF the prop is the correct diameter, then pitch the prop to maximum boat speed at manufacturer's, (Universal's), recommended sustained cruising RPM... not at maximum engine RPM. This may take several tries (fortunately Maxprops are adjustable).

The goal is best engine efficiency AND best prop efficiency. Are you using a reduction gear? Spinning the prop overly fast, decreases the work it does per revolution, while increasing engine RPM - you then reach a point of diminishing returns.

As an aside, the older rule of thumb was one HP per ton. Most folks may want more than that these days, but 4HP per ton for a displacement hull should usually result in an engine providing more power than can actually be used. Therefore, if you have an engine approaching 4HP/ton, look elsewhere for your problems, i.e.: propeller diameter, pitch, and prop RPM.

Robert Miller

[water addict]

32 hp should be plenty for that boat. Your engine should likely run at about 2200-2400 rpm at a fast cruising speed. I would suspect that your mechanic didn't do such a good job with the prop pitch. Also, for a boat that size, a reasonable fast cruising speed under power would be in the low to mid 6es, not 7.5 knots. A 7.5 knot cruise speed under power is what you would expect for something in the 40-45 foot range, unless it were a motor-sailer.

[FavoriteSon]

Hi yall,

so i was sitting around working on a 45 foot bermuda yawl. I think that a BMW D-50 engine. it runs at 45 hp 3200rpm. is this a good choice?
also i was checking the BMW website and there is no mention of marine engine. Does BMW still make marine engines?

[gonzo]

Hull speed for a waterline of 30' 6" is 6.6Kt.

[RDKinard]

Very interesting. US Sailing's Performance Program predicted boat speeds up to 7.5-8 knots in 20 knot winds. The program must be violating theoretical boat speed for that to happen.

Maybe our expectations are too high, based on everyone's comments.

I'll have the mechanic increase the maxi-prop pitch from 12 to 14 degrees to see what effect it has.

Thanks,

Captain Rich

[gonzo]

7.5-8 kts gets you into the semidisplacement mode. It depends on hull shape and displacement.

[Jim Herbert]

I guess it depends on your point of view. Smaller engines can extend the cruising range if speed is not important. I am rebuilding a "35" hp 2002 Universal M35B (Kubota) for my Islander 36. To attain hull speed I would need 55hp, the size for later model years. I removed a 16 hp Palmer.

The "Mother" of all amatuer boat drive train calculators is called "Prop King" and is available if you search the web for "prop king surfbaud". The authors point of veiw is that you should be able to drive your boat at hull speed with a 3 blade prop for lee shore situations. My point of view is that I couldn't afford a $10,000 motor when I could get one to rebuild for $750.

[Jim Herbert]

I have been unable to contact Mr. jimdwood who e-mailed me for the new web address for "Propking". If your out there jimdwood, here it is: www.electricboats.co.uk/surfprop/

[Ssor]

I sail in a 4 1/2 Ton Islander 30 repowered with a 10 horse volvo diesel w/ 3:1 reduction gear turning a 13x15 three blade prop I have no trouble reaching hull speed at less than WOT. If you consider that you can not reasonably exceed 7 kts. then turning a large prop at slow speed will be much more efficient than turning a small dia. low pitch prop at high speed. Pushing my boat past hull speed only digs a hole in the bay and increases the size of my wake.

[bobothehobo]

Wow, check out all the Islander owners....I sail a 1969 Islander 37, 30 hp engine, cruise around 6.5 kts....I believe hull speed is around 7.6 kts....

[Ssor]

Hey Bobo, Mine is 1968 30 ft. aquired in 1989 completely rebuilt inside and outside launched in 1999 Sail number 25 built by wayfairer yachts.

[Ssor]

I just did some figureing and came upwith these numbers; If someone were towing another boat and the strain on the tow line was 550 pounds and the speed was six kts. the energy required for the tow would be ten horse power.