Beck boat rebuild needing help

im currently in the process of rebuilding whats know as a beck boat. its a 16' fiberglass boat that was originally designed as a commercial sea trout trolling boat. they used a 16hp single cylinder briggs and Stratton engine for power directly coupled to the driveshaft. (no neutral and no reverse) I had a 10 hp yanmar diesel from a previous sailboat project that never worked out so when I got the hull so I decided to put this engine in the little boat because the stringers were all rotten and the engine was shot anyway. since then ive gotten the boat in the water a few times with the diesel in it and have started trying different propellers. I have tried a 14x10 3 blade and the engine revs up like it barely has a load. I borrowed a 16x16 and tried it and the engine made it to 2000rpm max and the hull was plowing quite hard as if it was trying to get up on plane but just couldn't quite make it. so I went and bought a used 15x16 because the general thought seemed to be that dropping an inch off the diameter would increase rpm by 400ish rpm. seeing the power curve for my engine it appeared that an additional 400-600rpm would put it right into the range where she starts making more torque and possibly be just enough to get it on plane. however as she performed exactly as she did with the 16x16. 2000 rpm and plowing trying to plane. (about 7-8mph) any suggestions as to what I should try next? I could have the prop repitched 2" down but that cost 100 here and that's more of a guess if that would even work. below are the specs of the boat and engine

length 15'9"
width 5'6"
approx. weight 1000lbs
slight v in the front and mostly flat toward the back
1" diameter driveshaft (I have an adaptor to use 1-1/4 props)
yanmar 1gm10 engine
1 cylinder diesel
3.2:1 gear ratio on the transmission (sailboat engine)
10hp 3600rpm max
8hp 3400rpm continuous rating

 

What is the shaft angle? It sounds like you're just too under powered. Even with a perfectly sized prop, it'll be tough to get a 1/2 ton boat with you, Fidel the wonder dog and a cooler full of beer up on plane, given a straight 10 HP diesel and it's related drag.

 

I'd have thought the only way to get a 16 foot boat to plane with a 10 hp diesel would be to wait for a tropical cyclone ( hurricane over there ) to arrive, then surf in !

 

The shaft angle is about 12 degrees and the thousand pound weight is including me and fuel and then I added in some to account for fishing gear and stuff. The bare hull with stringers and floor is about 240 lbs. I was kind of thinking the hull should be up on plane at around 10-12 mph because the bottom is so flat. Looking at some of atkins design boats unless he was incredibly generous with their speed estimates it didn't seem like my setup was too far off of being plausible. The majority of speed calculators seem to be putting my boat closer to the 15-16 mph range also uness them and Atkins use the same formula and In real life it's totally diffrent...

 

At 12 MPH and assuming a LWL of 15', you're doing 2.0 S/L ratio, which is not a full plane, but semi plane (just barely) and this is a zone of operation that's difficult to attain with the typical shaped used on full plane powerboats, particularly bunt bowed ones. To be in full plane mode, you'll need to pushing well into the mid and upper teens, if you expect to sustain this speed. Simply put running at these low semi displacement speeds, you can easily get slapped off plane by a wave, wind, contrary currents, etc., so you need the power to push through.

Unless the Atkins designs you've been looking at, are very nearly identical to the shape, weight and power of your boat, direct comparisons are essentially meaningless. Lastly, a 12 degree shaft angle means you're losing a lot of the limited available thrust, so your HP figures are further reduced, over the usual loses associated with a straight shaft setup. With a 15 HP engine, you'll run in the mid to low upper teens, assuming the resistance and loses with your setup.

Pictures (profiles) under way at WOT will typically tell everything you need to know, about where you are in terms of being fully up on plane. I'll bet your midship wave train is still attached, as is the stern wave, with a significant trim angle at WOT with your setup.

 

If it is only pulling 2000 rpm ( how did you measure that ?), clearly it is going to be useless, unless you get the (much) lower pitch propellor, especially if it produces the 10 hp @ 3600 rpm.

 

oh ok yeah the atkin designs are substantially different from my hull. as for the rpm I have a tach hooked up so that's where that number came from. I never really thought about the shaft angle either. and yes the 2 wakes are still attached so I guess im not as close to being on plane as i thought. I guess ill take the prop to the shop on Tuesday and have them repitch it to see if I can get my rpm up. if anything ill try to get the engine running in the rpm range that it needs to be running in and if it planes then great and if it dosent then ill have an almost planning boat haha. would you agree that 2" off the pitch would be the right direction to go to get the rpm up or should I have them take an inch off the diameter and 2" of pitch off it... or should I look into a new prop entirely... below is a picture of the bottom of my hull by the way...

 

Seemingly, small decrease in pitch won't be enough, there is a huge gap between 2000 and 3600 rpm.

 

Oh. I was thinking 2" because if that bumps the rpm up by say 300-400 then the engine will be making a lot more torque in that range and it only goes up from there. Or does it not work like that and I need to look at taking an inch off the diameter also...

 

Instead of "playing" with this, why don't you log onto the one of several sites, that can ball park your prop for you and work from there.

 

Michigan wheel said I needed a 15x15 and another said 15x12 and anothe said 15x14. So that's how I got in what I thought was the ballpark... It seems to be a large ballpark tho...

 

At 3.2 to one gear ratio, you may not be able to spin the prop fast enough to get much planeing action. At a thousand pounds and 10 HP the 100 pounds per HP is at the low end of the performance scale.

 

I use Michigan quite a bit. What speed did they say you'd get with the 15x15?

 

Michigan said I should get about 14mph with the 15x15. I did manage to solve the mystery as to why I didn't pick up any rpm going from the 16x15 to the 15x15. the 15x15 prop I bought was used and I didn't think to look for cup on the blade and yesterday I noticed it was a cupped blade. I took it to a local propeller shop and spoke with them and they recommended me to leave the cup but take out 3 inches of pitch and see how it does then. if that's not enough then we will take the cup out and see how it does and if its still way out then possibly take a half inch off the tips of the blades to drop me down to a 14x13 which should put me close hopefully. that way ill be about less than $400 into this propeller. (figure $100 each time to have it worked and I paid $100 for the prop) and that's still being generous because repitching is only $85 and im sure everything else will be less.

 

Well I got the prop back and mr efficiency was very right. It wasn't enough pitch. They took out 3" of pitch and left the cup and I picked up 600 rpm so I'm now spinning 2600 rpm. I went to have the propeller guys do some more work on it and when I mentioned that I had someone recommend a much smaller prop with more pitch he went to the back and came back with a 4 blade 13.25 x 16 prop and said I could try that one and see what it does. So tomorrow I'll be getting another bushing because the 4 blade is for an 1-1/8 shaft and I'm running a 1" shaft with the bushing for 1-1/4 props. At least I'll have the whole bushing set now I guess haha.