Surface piercing prop help!

[tyst67]

I just finished my new lightweight homemade craft and it weighs about 180lbs! The motor is a 30hp mercury! My prop is a 18p Merc chopper 2! Before, on my last boat which was a lot heavier but same design, my propshaft was 90mm below the bottom of the boat! Everybody would tell me to bring my motor up until the propshaft was level to the bottom of the boat! So this year I built a higher transom to do so! I added 5 inches of setback as well! The new boat has a tunnel and the propshaft is level with the bottom of the boat, not the tunnel! My motor sounds great and a lot less of my running surface is on the water on full plane BUT my speeds are slower than before on a much heavier boat! Is it possible that now the prop is slipping more and not getting the proper thrust? I feel like I want to lower the motor but I hate to cut the new transom! Please help!

[FrenchPhil]

Choppers are not surfacing props so I'd get my hands on a cleaver like a 10 x 14ish for starters. What max speed & rpm did u reach? Show us some boat & setup pics.

[Roflhat]

Choppers are surface piercing props

[omcstratos]

You may want to see what Ron has to offer.

[fs5]

you may have to get some cup added mate.
how many revs and what speed?

[tyst67]

you may have to get some cup added mate.
how many revs and what speed?

I don't have a tach but it's screaming! The speed is 55.5 mph! Last year it was 58.7 mph on a much heavier boat and my motor was 90 mm lower

[Cajun Special]

With the new added setback you could be stern heavy (center of gravity too far back). Just a Thot. It happened to me when I took the hook out of my hull. Majority of my setback was no longer needed

[fs5]

when I was running choppers on my tunnel hull ,with the motor raised to the max ,I had to get some cup added.it made them a little faster ,probably a couple mph ,not a great deal more.
I think the choppers are only 10 1/4 diameter ,a bit to small i think..
you may need a larger diameter prop now with the extra engine height .i'd try getting some more cup added first though.
can you post a pic of your prop?

[tyst67]

57314 it's the prop on the far left 18p mercury chopper 2

[omcstratos]

How does that semicleaver do on it?

[tyst67]

How does that semicleaver do on it?

It is about the same!!! They are both good working props but my motor was too high! I cut my transom down 2 inches and gained 1 mph! I think I have to drop it farther!!! I am still 50mm above where I run last year an still 2 mph slower!

[omcstratos]

I've kept up with a boat on screamandfly thats a 13ft tunnel with a yamaha v4. He runs et props because his motor height is too high for a cleaver to grip.

Can you squeeze a bigger diameter prop on?

[ice_spy]

On a tunnel, a surface drive prop should be level with the bottom of the tunnel plank. Remember the more set back the more you have to raise the engine as the water rises the further back your motor is from the transome. Ideally, an adjustable motor jack plate that also gives you set back is the best way to adjust engine height with different props. Choppers are surface drive props. 18 p would be far too much pitch to drive submerged for a 30hp. Also these props are designed to run out of the water not under. Tip cup will reduce some prop slip. You are probably running the prop too low, is venting the prop under the water and you are just getting a lot of slip. You need to 'free' the prop and gearbox to unleash the surface drive props potential.