13' displacement hull, prop selection

[Daveyduck]

I've got a 13' duck boat that is a displacement hull. It's a very safe boat, USCG approved, can handle some rough water.

I have a 2011 Mercury 15, 4-stroke short shaft on the back bolted to a CMC power trim plate. I also run a Hydrofoil on the motor to help get the bow to come down a bit when running in calm water. If the water gets rough, I can just back it down and bit and she handles great.

The boat does 11mph tops. If I'm running against the current, it can go as slow as 8. The problem is I'm running wide open all the time. I can't go up to a 25 because the boat is rated for 15hp max. I have thought about a 25 2-stroke and may borrow one to test out, but I really like my 4 stroke Merc. Electric start, runs quiet, no mixing oil and it's new, still under warranty.

I currently have a stainless steel 3 blade prop in 7" pitch on the motor. The factory 9" pitch prop was horrible, the motor couldn't even get near the rpm range. I tried a 4 blade prop, that was terrible too. The stainless 7" prop seems to be the best out of what I've tried. The motor still runs below the preferred rpm operating range on the top end. It just can't spin fast enough to push the weight of the boat. When under power, the transom just squats so bad and the bow comes up pretty high. My hydrofoil/SS prop combo is definitely closer to good performance, although I'm wondering if there's anything else I can try to pick up a bit of speed. Any suggestions? I'll try and attach a pic of the boat.

 

[smokeonthewater]

Re: 13' displacement hull, prop selection

you still have too much prop if you can't get into correct rpm range

you are GONNA be running wide open though and the boat is GONNA act like you described because you are pushing a displacement hull at over displacement speeds... back it off to about 5 mph if you want to cruise easy

If you want more speed get a planing hull

 

[jestor68]

Re: 13' displacement hull, prop selection

It appears that the lowest pitch available is 6"; which should raise your WOT rpm by a couple hundred.

What rpm you're turning now will determine if that would be enough; the range being 5K to 6K.

 

[steelespike]

Re: 13' displacement hull, prop selection

Is your rpm a secret?What does the boat weigh.Some pictures would help.As suggested a Displacement hull is not intended to plane.
Max efficient speed would be about 6 mph and it should do it very efficiently.
You could probably run about 6 hp and not have to open the throttle.A 6 hp 4 stroke is about 57 lbs A sizable improvement over your 135 lb 15 hp.And you wouldn't need to carry so much gas.With that weight gone it may not squat so easily.A 6hp wide open might make 8 or 9 mph. and use only 6/10s of a gallon per hour.instead of 1.5 of the 15.And at reduced throttle less than that.
The hull is probable really rounded and has a lot of fore and aft rocker.
If you must make it plane You might try adding over size trim tabs not as tabs but to increase your planing surface at the stern.
and give the hull a flatter run aft.
There is no doubt that your foil may add lift but it also is adding a lot of drag.