Optimum Operating Range ?

Hello Folks

I'm doing the homework on a new build (a lightweight planning commuter, 4400 lbs displacement, 30' lwl) that will mostly operate at about 12 knots requiring (theoretically) about 30-35 HP.

The top end speed required is 20 knots, and a 90 HP might just deliver this. Not surprisingly all the sales folk are steering me to their 90 or 100 HP 4 stroke options!!

I have operated diesel engines in a variety of circumstances both marine and land based, but very few "gas" engines so I'm trying to gain a better understanding of the wider considerations.

My question is - what are the implications of operating petrol outboard engines at 1/3 of their capacity for most of the time - say 80% of the time, but occasionally requiring say 120 minutes at maximum output (WOT). Will I be able to "prop" it effectively? Will it dramatically reduce the service life, is a 2 or 4 stroke more suited (why) etc ??

Given the parameters of the design I'm pursuing is there a better way ?? For example I know from operating diesel tractors and trucks offroad that I want to be pulling at engine revs where my torque goes up as revs start to drop, same on my displacement sailboat. For example might used twin 2 strokes of 50 HP each be a better solution.

All thoughts welcome
Foster

 

Sure wish you'd gotten some responses as I'd like to have heard them.

May I offer some guesses?
I think answering your question on a broad basis would not be possible as best scenario would flip back and forth.
One question that a really experienced OB man would know is how gracefully each runs at near WOT. Off the top of my head I'd say the two stroke would do better at that but it's possible a four stroke could do as well. Valves may limit the four stroke somewhat due to heat.
The OB by it's design has the power plant all the way aft so the aspect ratio of the hull may be a big variable. A short wide hull would be at a serious disadvantage at slower speeds so a high aspect ratio may be required for the flexibility of running speeds.
I think where I'm going is that it's possible the hull design may be more important than the flexibility of the engines. And getting back to that running the engines at near but not WOT may indicate either are fully capable.

 

Can you post your calculations? I think there has to be a mistake somewhere to show such a low power for the speed.

 

Not if he is thinking of a 30ft powercat....

Richard Woods

 

A commuter implies a monohull.

 

The range 12-20 knots should be OK with the 4S outboard, but you would want one with the larger gearcase reduction/prop diameter if 20 knots is flat-out, for best results.

 

what are the implications of operating petrol outboard engines at 1/3 of their capacity for most of the time - say 80% of the time,

Gasoline engines suffer less from low power take out than diesels.

My concern would be 2 hours at full throttle , many outboards are not built around engines with that stamina.

The ability to pull back say 10% of RPM might have the engine last longer , tho it may also require a larger engine.

 

I would not run any engine at wot for more than a few minutes. 2 strokes will die quickly at wot with a bit of load on . I always take mine to wot then back off a few hundred rpm till i hit the sweet spot where it sounds its best. Usually around 4500 rpm.