Prop size and factory spec.

I have a Suzuki DF115A on my planing boat. Suzuki recommends an operating range of 5000-600o rpm. Current prop size is 19" pitch that gives a WOT of 5800 rpm.

For occasional, long distance trips at low -legal- speeds, I'm considering fitting another prop of 23" pitch that will -propably- reduce WOT to 5000 rpm. The idea is to reduce rpm to lower the noise for a more comfortable ride.

Would this be harmful to the engine ('lugging') despite WOT staying within official factory spec ?

THANKS,
V.

 

Your boat may struggle to plane with a 23" prop, but I'm not really sure what your plan is here, what is the cruise speed you have in mind ? Just about any mechanic you ask, will say prop toward the upper end of the RPM range, but you really need to consider what the worst possible scenario would be, in terms of loads carried, water conditions etc that would make the 23" prop a liability.

 

What speeds are you currently achieving at 5,000 rpm and at 5,800 rpm?
Or what is the minimum revs that you currently need to stay on the plane in flat water with an 'average' load?
Rather than installing a propeller with more pitch, which will force your maximum revs down, might it be easier to just say that you will cruise at lower revs anyway, such that the noise level is acceptable?
If you decrease the revs, you increase the torque for the same power - can the engine cope with this?

 

You would be overpropped with a 23 inch prop. To keep from overloading you could go as high as a 22" prop and that would still keep you at the bottom of the correct range for the motor. In simple math, to reduce the prop rpm to 5000 from where it is now (5800) and maintain the same speed you want a prop with 16% more pitch, which would be a 22 inch prop. If your speed was the same and you had a 22 inch prop you'd be right at 5000 rpm at max speed. Since props typically come in 2 inch increments you can go with a 21 inch prop and that would reduce the rpm's by 10%, and of course that would slow the motor down a bit, from 5800 rpm down to 525o rpm. It would be a bit more quiet, but don't expect much. Remember that motor noise is a function of speed and load. Slowing it down, but keeping the load the same means that you'll reduce noise some, but not a whole lot. Also remember that pushing the pitch up means that it will be harder and take longer to plane off. Hard to say that it's worth the effort, but that's about where you are.

 

Backed off, in rough conditions, is where too high pitch really hurts, most offshore boats tend to go a size lower than would be ideal for good conditions. Motors are strained by running at hump speeds, and overly strained if the pitch is too much.

 

Thanks all.
I want to achieve a cruising speed of 9-1o knots, which is the max. allowed speed on many of the inland rivers/canal around here.
Today, at this speed (in between hull and planing speed) engine runs 3000-3500 rpm, producing a lot of noise/resonance.
Hull speed of my boat is 4-5 knots at 1500 rpm with the 19" prop.
Boats planes at 4000 rpm, at 18 knots
WOT is at 5800 rpm, doing 30 knots

V.

 

That is just a bad speed to want to travel at, the boat will nose high on the "hump", and you don't do your motor any favours by increasing propeller pitch in that situation. If you think noise is a problem, maybe an engine cover will assist.

 

Thanks, that's what I feared....

What's your view on adding a kicker engine?

 

That will only give you the 5 knots, and probably be noisier than your 115 while doing it. There really isn't a lot you can do to increase the speed up to the maximum allowable speed, without being in that transition phase.

 

Vronsky, be aware that while there is a speed limit on your inland waters, there might also be rules regarding the amount of wake or wash that you create. On inland waters this is usually a more important factor than speed. And you will be creating a huge (relatively) amount of wake while travelling at your speed limit.

 

Yes, it's getting more clear to me now.
....seems I have the wrong boat slow motoring

 

If you are predominantly going to use your boat on inland waters, then it is a complete waste of time having a planing hull - a fuel efficient relatively narrow displacement hull would be much more suitable.
Or (even better) a displacement power catamaran - this has two very narrow hulls, hence you should still be able to achieve 9 or 10 knots without too much wake.

 

I understand from above that cruising at 10 knots my boat is at transition/hump speed which creates extra strain on my engine.
Suppose I still want to continue at 1o knots, despite the noise, would a lower pitch prop (19" -> 17") perhaps even be better here, as it will reduce strain ?

WOT (19") now is 5800rpm, rev limiter intervenes at 6300 rom

 

A lower pitch prop will make the engine rev higher at the same speed. There will be less load on the engine, but won't make much of a difference. The "transition" speed is the worst operating range for the boat. The solution is to either slow down or speed up.

 

What bajansailor says about the wash, and how it will be substantial, could indeed attract disapproval. Certainly this not the ideal boat for the purpose.