What do People Use for Engine/Propeller Sizing

[Seaweed827]

Hey Everyone,
What site or how do you get the information you need when sizing an engine, transmission/gear ratio, shaft, propeller etc. for a sailboat?
Do you use a certain website, engine dealer, prop shop etc?

[TrentePieds]

Interesting question :-)

Not so much in terms of any answers you may elicit, but rather as to why you would ask it at all. Are you designing a boat?

TrentePieds

[Seaweed827]

Replacing engine on my Catalina 400, currently has a Yanmar 54hp
Would like to get better speed/power while motoring.

[skipperpete]

For small engines up to 400 hp the Yanmar installation manual has all the answers for their engines and reversing gears, they list all the details from 1GM10 to 6CX-GTE…. Literally every tiny bit of info. For Australian commercial vessels under survey the calculations are done by a naval architect within an established set of requirements outlined in AMSA guidelines. Most engine manufacturers have technical data available online and there are numerous sites for prop matching calculators and shaft sizes.
Regarding your need for extra speed, that is limited by the “hull speed” of your vessel if its a displacement hull and after a certain point it makes no sense to add horsepower. Different for planing hulls though., and maybe foilers too in the near future.

[TrentePieds]

Well, what is you target STW? What do you mean by "more power"?

TP

[Scubaseas]

54hp in 20000 lb. boat is not really under powered. A displacement hull won't go faster than hull speed (unless is planning), Adding more power will just make the boat "squat" down more in the water. The engine manufacturer or dealer should have any info you need.

[deblen]

I will toss this into mix:
Some sailboats act underpowered,especially during low speed/docking maneuvers,because their propeller is chosen for it's "less drag under sail" properties.
You must decide if you are willing to give up approx 1/2 kt of top speed under sail,due to prop drag-
in return for being able to dock & maneuver confidently & safely plus be able to accelerate/stop quickly and power thru headseas when necessary.
Cheers/Len

[boatpoker]

Originally Posted by Seaweed827

Hey Everyone,
What site or how do you get the information you need when sizing an engine, transmission/gear ratio, shaft, propeller etc. for a sailboat?
Do you use a certain website, engine dealer, prop shop etc?

I called the guy that designed my boat

[Kd9truck]

Hi, Just looked at your Catalina 400 MK II ? on SailboatData.com your hull speed is 8.1 knots with a YANMAR 4JH3BE. 8.1 knots is the max of what you will get if your motor is at 3k rpms with a clean hull, prop and rudder.
Have you looked at the polars for your vessel? Your polars will give you the max of what you can get under sail. This will be a bit harder to find I don’t think you can do better than the listed hull speed (8.1 knots) with your vessel.
Think of hull speed as terminal velocity for displacement hulls, if you have the intended motor (above ) and a fixed prop that came with it you are going as fast as the hull design allows.
*You can go faster than hull speed if you sail and motor at the same time but you will not beat the polars numbers unless you add sail to you vessels plan.

[fourlyons]

Well, to actually answer your question, I used Dave Gerr's book, The Propeller Handbook. I have found it to be very accurate.

[skipperpete]

At the top of this thread there’s an ad for EWOL propellers, a really good solution to the propeller question and since they’re a forum sponsor I’m confident that they could give you a qualified opinion if you contact them. My maxprop classic 3 blade is very worn and it’s replacement will be an EWOL rather than get the maxprop rebuilt.

[FPNC]

By hull displacement, (~20,000 lbs) you only need between 40 and 50 hp to get 'maximum speed'. The original propeller on the Catalina 400 was probably an 18" diameter, 3 blade. An 18" dia 3 blade propeller will only transmit a maximum of about 50 hp to the water in an efficient manner. There is no need to over power your propeller in a displacement vessel. Find out your propeller specs I.E., Maximum diameter that can be turned or maximum diameter that you are willing to fit, and call a decent prop shop. They can give you the answer.