Faster Prop for Yanmar YSB8 Diesel on Cape Dory 27

[Sailor_Grant]

I have a Cape Dory 27 with a Yanmar YSB8 on it and right now it goes quite slow. The boat only goes 3 knots with the current prop and need it to go faster (and I think it should).

What prop should I use to speed up the boat?

For those not familiar with Cape Dory 27, the boat has a full keel and is of medium displacement (7500 lbs).

https://sailboatdata.com/sailboat/cape-dory-27

[pilott]

Unfortunately, a new prop may not be the answer.

The general rule of thumb for sizing a sailboat engine is 2hp per 1000 lbs of displacement.
So for 7,500 lbs 15 hp is indicated. This isn't cast in stone, of course!

The YSB8 is, AFAIK, 8hp. So it may be that the engine is under-power.

For sizing a prop, you'll need to know your transmission ratio.
There are many websites with on-line calculators. Here's one:
https://www.miwheel.com/inboard-propellers/prop-it-now/

There's also the option of having the pitch of your existing prop adjusted to make it deliver more power

Good luck, and please let us know the outcome

[jamhass]

Is the engine able to reach it's rated RPM at full throttle? It should be able to.

If it reaches full rated RPM at partial throttle, you are probably under-propped (too low pitch)

If it cannot reach full RPM, then you may have prop issues, or other issues. For example, is the bottom clean?

[Wotname]

Originally Posted by jamhass

Is the engine able to reach it's rated RPM at full throttle? It should be able to.

If it reaches full rated RPM at partial throttle, you are probably under-propped (too low pitch)

If it cannot reach full RPM, then you may have prop issues, or other issues. For example, is the bottom clean?

Jamhass has it right!

First you need to know what rpm you are currently getting at WOT (wide open throttle) with a clean prop, clean bottom and in flat calm water.

This is the starting point to determine what changes you can make to your existing prop.

Ideally you should reach hull speed (with clean prop / bottom / calm water) well before maximum rpm but this depends on the horsepower you have available. Put another way, if you reach hull speed well before maximum rpm, you have some reserve power available when you go to WOT to cope with the additional drag caused by increased wind or wave action.

Here is another on-line prop calculator that may be helpful https://www.vicprop.com/calculator.htm

[FPNC]

Propped correctly, that engine with a 2:1 ratio gear should give about 5.2 kts. Reasonable LWL speed would be 5.8 kts and a 13-14 hp engine would give the 12 hp you need at the wheel to make it go that fast.

Your going to need something like a 12 x 7, 3 blade propeller. Take your engine and boat specs to a good prop shop and ask them to run the numbers and recommend a propeller. Since the prop is in an aperture, it would be good to know the maximum size that can be fit. Which is probably 13".

[Sailor_Grant]

I realized the problem stemmed from the clogged exhaust mixing elbow. After replacing that, I was able to go 5 knots with no problem.

[Fore and Aft]

That’s a handy thing to know Grant, I would never have guessed that was the issue.
Cheers