Glen-L.com

A question for all you experienced builders:

I'm building a planing electric runabout (see post on 16' mahogany runabout - electric), and as you might guess efficiency is going to be a very important part to ensure a decent amount of fun on the water. Based on experience with other small boats, a ton of online research, and crouches formula I am relatively confident I can operate at planing speeds for well over an hour...so here are the important questions.

1. How much does your boat weigh?
I have estimates of how much mine will but was wondering if you zip, ski king, crackerbox owners, etc.
have an idea of how much your boat weighs wet and/or minus drivetrain. I estimate a wet weight of
my boat to be around 1000 pounds minus passengers.

2. How much horsepower does it take to go a specific speed?
This is a tough one with gas boats because generally only the peak horsepower is known which can
assume will correlate with top speed. Problem is top speed is generally not a good indicator for
extrapolating cruising speed power because the speed may be limited by running attitude, propeller
design, etc. Does anyone know gph burned at specific speeds? Knowing this might give better
extrapolations...

3. Hull number?
I've been using crouch's equation with a hull number around 200 for my flatbottom/shallow-v
runabout. I chose this based on some boston whaler owners finding that 180-190 matches their
deeper vee hull designs pretty well. It will probably end up being higher than 200.

You all have probably seen this mercury website that uses crouch's number to predict speed:
http://www.go-fast.com/boat_speed_predictions.htm
I think their rule-of-thumb numbers for different boat designs are too high.

If I use 1300lbs (two passengers), 15hp, and a hull number of 200, I get speeds around 20mph, which is a respectable cruising speed on the water in my opinion.

Any thoughts or experience?

My malahini came in a little heavier at 1380 lbs with a 26ish gallon fuel tank and motor installed. Still have some fine tuning to the motor for performance but I get a maximum (GPS) of right about 34 mph (flat run 2 minutes averaged north then south). Power is a 1983 Evinrude 70 HP weighing in at 237 lbs. Had me, 4 big boys, and a full tank on board last year and still got up to about 31-32. I used heavier wood species (White oak for framing material, meranti plywood, and 4/4 Kahya for coverboards/trim). With everything trimmed right with the motor I think I'll probably only get to about 35-36 which is still fast enough for this guy.

Also, once on plane I can reduce speed down to about 15-18 mph and still stay on plane. Comfortable crusing is right around 28.

_________________
Bill

I told my wife we needed a three-car garage for my projects...she told me to ask her for permission next time before I buy a house.
http://www.unitybuild.net

Bill, thanks that helps! I plugged your numbers into the calculator and got a hull number of around 180 assuming the 4 big boys weigh 200 pounds each. That is a respectable number and in reality the number is likely higher if everything is not tweaked for top speed. I bet if you knew the amount of power you're using at your comfortable 28mph cruising speed you would be using less than 55hp, which is what a 180 hull number would predict...what do you think?

Also Bill, I'm glad to know you can plane down to 15ish mph...I was wondering what the lower limit would be and my hull shape is similar to yours with a bit less weight. Even using your number of 180ish for hull number I should only need about 10 shaft horsepower to go this speed...that would be great!

I will say I need more to get on plane but after I'm up I can power down to that range when only carrying about 5 gallons of fuel, just me (and maybe one other) on board, and the right water conditions. I'd say about 75% is about right for crusing for me with my current set up.

_________________
Bill

I told my wife we needed a three-car garage for my projects...she told me to ask her for permission next time before I buy a house.
http://www.unitybuild.net

Nice. People have found the formula to work within single digit percentage accuracy once hull number is obtained (below 50 knots at least). Anyone with an inboard have weight and speed data? Maybe a crackerbox?