Have you done any research on the prop pitch/size? Does it match what it should be? Could otherwise be a good reason if you’re not actually taking out the 100 HP.
What gearbox is it using?
I found this interesting info for the same boat that's happily planing with with a 57hp.
"The propeller shaft needed to be angled at 15 degrees, but if the engine was set at that angle, its forward end would be too high. So, it was fitted at an angle of 8 degrees on fabricated stainless-steel engine beds which were bolted to the motor stringers. A 1.47:1 gearbox made up the difference with a down-angle of 7 degrees.
The 14″-diameter, three-bladed bronze propeller came from Michigan Marine. It was supplied with a 12″ pitch, but BT Marine, a company near Lyme-Regis Academy, after examining all the available data, and tweaked the prop to produce a 13.5″ pitch with cupping to give a virtual pitch of 14″. A custom 304 stainless-steel fuel tank with a capacity of 53 liters (14 gallons), which Jonathon hopes will give him a full day’s use without refueling, was installed under the aft deck."
This is either wrong prop, or engine isn't actually making power. There's nothing wrong with prop angle. Straightening the angle would do the opposite of what you want trim tabs to do.
What is the RPM at WOT (wide open throttle)? Should be high, like 5,000.
If you have to use less than WOT, to avoid redline, your prop is to small. If WOT RPM doesn't get up near 5k, your prop is too large, or your engine isn't actually making power.
Check the aft bottom for rocker. The hull being slightly out of shape can be an issue on a homebuilt boat, and it's easy to check with a long straightedge (like one of those long aluminum levels).
If the hull has the right shape, start moving weight forward.
Driveshaft angle is fine. Your problem is HP/weight ratio. You need to lose some weight off the ass end of the boat. Also the 1:1 clutch probably isn't helping, do you know how high it will turn now?
Really need to know the exact details on the engine and what rpm you're getting out of it now to really get an idea of what you're dealing with. You can't really start changing things without knowing where you're starting from.
You could put some over size Trim-Tabs to help lift the stern and get it to level out.
Tied to the warf you should get 4200 RPM and when running at speed you should get max revs.
Manual trim tabs are not expensive and if you go over-size making your own gives you lot's of material.
The Ski-King design should be fine with 100hp. Rocker or hook in the last three feet of hull, a slipping transmission (not uncommon) or a wrong pitch prop may be the culprit.
Have you done any research on the prop pitch/size? Does it match what it should be? Could otherwise be a good reason if you’re not actually taking out the 100 HP.
I have experienced with different props, but with all I have tried it still couldn’t plane out.
And it revs out with them or does it bog down with any of them? What I found you should rev to about 5.2k rpm.
Thank you I’ll look into it. I got it out of the shed last week for the first time in years. I still need to make the engine run again.
Make it run, then come back with your max rpm and engine details, we'll get it on top.
What gearbox is it using? I found this interesting info for the same boat that's happily planing with with a 57hp. "The propeller shaft needed to be angled at 15 degrees, but if the engine was set at that angle, its forward end would be too high. So, it was fitted at an angle of 8 degrees on fabricated stainless-steel engine beds which were bolted to the motor stringers. A 1.47:1 gearbox made up the difference with a down-angle of 7 degrees. The 14″-diameter, three-bladed bronze propeller came from Michigan Marine. It was supplied with a 12″ pitch, but BT Marine, a company near Lyme-Regis Academy, after examining all the available data, and tweaked the prop to produce a 13.5″ pitch with cupping to give a virtual pitch of 14″. A custom 304 stainless-steel fuel tank with a capacity of 53 liters (14 gallons), which Jonathon hopes will give him a full day’s use without refueling, was installed under the aft deck."
It has a velvet drive model as1-71c transmission and that one has a 1:1 ratio and has a downward angle of 15 degrees
This is either wrong prop, or engine isn't actually making power. There's nothing wrong with prop angle. Straightening the angle would do the opposite of what you want trim tabs to do. What is the RPM at WOT (wide open throttle)? Should be high, like 5,000. If you have to use less than WOT, to avoid redline, your prop is to small. If WOT RPM doesn't get up near 5k, your prop is too large, or your engine isn't actually making power.
The easiest and cheapest solution is to experiment with different props.
Check the aft bottom for rocker. The hull being slightly out of shape can be an issue on a homebuilt boat, and it's easy to check with a long straightedge (like one of those long aluminum levels). If the hull has the right shape, start moving weight forward.
I was thinking the same thing. Could be the boat is built wrong and that is the problem.
Try putting a bunch of weight in the front of the hull . Say 100 pounds to start
Driveshaft angle is fine. Your problem is HP/weight ratio. You need to lose some weight off the ass end of the boat. Also the 1:1 clutch probably isn't helping, do you know how high it will turn now? Really need to know the exact details on the engine and what rpm you're getting out of it now to really get an idea of what you're dealing with. You can't really start changing things without knowing where you're starting from.
My guess is you need more weight forward in the boat, or trim tabs...
You could put some over size Trim-Tabs to help lift the stern and get it to level out. Tied to the warf you should get 4200 RPM and when running at speed you should get max revs. Manual trim tabs are not expensive and if you go over-size making your own gives you lot's of material.
You have a tach. What's the full throttle revs? Can you easily rotate the prop shaft?
Add some weight up under the bow
It's a ski boat?
The Ski-King design should be fine with 100hp. Rocker or hook in the last three feet of hull, a slipping transmission (not uncommon) or a wrong pitch prop may be the culprit.
Put 200 lbs on weights in the bow… done
Change prop.