Engine RPMs vs. Hull Speed

[Bill Roosa]

So what I gleen from this is

a) Props drive boats when the wind dies.
b) Props drive boats when the wind is contrary or schedules need to be met.
c) everybody seems to want to go hull speed all the time the engine is on.

A questions to the group:
1) Do you plan on using the engine to get you through no wind conditions (maintain boat speed) or do you plan on a slower transit taking into account the inevitable no wind conditions?
2) Do you plan on using the engine to "get you home in time" or plan to leave sooner so you can sail home?

The reason I ask is I almost never (haven't so far with my current boat) use the engine to go hull speed because I plan for a sail not a motor. so I would think I would want a prop that maximizes (various forms, MPG, engine wear.....) my normal slower speed usage and not one optimized for pushing the boat at hull speed. FTR, I use 4.5 knots as a planing speed with a 7 knot hull speed boat.
Thoughts?

 

[petersea]

"based on that would you be good enough to tell me if there is a diff in a 2 blade 15/10 and a three blade 15/10 other than the extra blade ......"

Woody, the difference is the total surface area of the prop blades. A 15/10 3 blade will have 33% more surface area a then a 15/10 2 blade prop. The more surface area the more power required to turn it. The more surface area the more thrust or power (not necessarily speed) These two props sizes are not "equal". It is possible a good prop shop can change the pitch and maybe the diameter to the correct size needed (after getting a good calculation) but may not be cost effective.

 

[petersea]

"So how important is it really to set the prop to reach full RPM at full throttle? My data point says it wasn't important with a gas outboard. I'm thinking about using too high of pitch prop again because I like the lower RPM, lower noise, likely better gas mileage of having the prop pitch too high. I really don't need the peak power of the outboard (might of course imply I picked one with too much HP, the new one is a Nissan 9.8 HP - necessary to get two cylinders and electric start). "


Walt, I think it is very important that you be propped close to max engine rpm. Not exact but close. Over propping an engine will lead to shorting engine life possible engine overheating and other problems. In an extreme case it could cause a total engine failure. Check you outboard manual Honda will give you several prop size option based on if you are looking for speed or power. In your case you will want the power or get a good prop shop to run the numbers. Michigan Wheel has a prop calculator on it's web site to use also. Under propping can be just as bad as over propping as it will give you excessive engine rpm, compare it to driving you car in second gear at highway speeds.. Over propping is a little like driving your car uphill in the wrong gear (forth instead of third) with your foot to the floor all the time.

p.s. Got to think that incorrect prop size would also void the warranty on your new engine.

 

[druid]

A questions to the group:
1) Do you plan on using the engine to get you through no wind conditions (maintain boat speed) or do you plan on a slower transit taking into account the inevitable no wind conditions?
2) Do you plan on using the engine to "get you home in time" or plan to leave sooner so you can sail home?

I CRUISE in my boat. Sometimes, especially for a daysail, I'm out to sail, and if there's no wind, I bounce around until the wind comes back or I have to go in. But for multi-day cruises, if I didn't motor I wouldn't get anywhere. A cruise up the coast is often 80% motoring. I've thought of just going out, sailing as far as I can in a day, and just using the motor to get in and out of the anchorage, but especially in August, I'd probably take a week just to get 50 miles from Vancouver. If there's wind, I sail. If not, I motor. And of course there's often 30-40 miles between anchorages, and most days not enough wind to sail 30 miles.

At one point I thought about selling the sailboat and getting a trawler-style powerboat, but realized "I already have one!".

druid