First Run Results... Now what?

**CodeRed225** · 06-01-2021, 07:30 AM

I am looking for some advice on what direction to go for prop selection with my newly rigged boat. Here is my setup. I have a 2072 MVT SeaArk aluminum boat. This is a very large, heavy aluminum flatbottom boat. It has a 4" tunnel in the back. I am powered by a 2018 Pro XS 115 with a 2.38 gearcase. I have a 5.5" hydraulic jackplate. It is tiller steer. I was told I should start around a 13.5" 17-19p.

The first run was with a 13.5" 19p in pretty bad shape. I bought it for $20 just to try to get a baseline. The boat would cavitate just about the time it was about to plane out. Nothing seemed to change this, weight distribution, trim, motor height.

The second run, I tried a 14.5" 24p stainless predator that I had laying around. I was shocked how well the motor handled it. It popped right up on plane. I got up to around 5000 RPM and around 36-37 mph. I started to raise the motor and trim to get more rpm, and while I could get the rpms up to 6000, I only got a top speed of 39-40. This calculated a prop slip of around 30%, which I was very disappointed with.

The third run, I borrowed a friends +/- 14.5" 24p 4 blade Trophy. I was thinking the 4 blade would help with the prop slip. The boat jumped out of the hole. Really good holeshot. It had similar to worse results on the top end. I could only get 38-38.5 and I got the rpms up to 6250 before I started dropping water pressure.

The boat is not yet fully rigged and if very light right now, I don't know the exact weight yet or cg. I had 12 gal fuel in the back tank, 0 in the front. I had two friends with that I had hoped would be able to move around to play with the weight, but didn't seem to effect anything.

I'm wondering which direction to go for prop selection. It would seem that I need a bigger diameter or more pitch. It just seems like I am already pretty far from the original recommendation. Maybe something with some cup would help with the slip? I have more access to bigger props so I am going to try a bunch this weekend. I welcome anyone's thoughts on my rig.

**XstreamVking** · 06-01-2021, 08:26 AM

Honestly IMO it's doing fairly well with the predator. But, you are getting a LOT of slip. For 115 hp on a 18' flat bottom that speed is respectable. Tunnels are gonna want a prop with a lot of cup. Predator makes a 4 blade that might be a good match.

**CodeRed225** · 06-01-2021, 09:15 AM

Thanks for your input. My goal for the boat is 50 mph. The way it pulled out of the hole with both 24s seemed very promising. Then the slip from 3/4 to full throttle was weird, then raising the motor and adding more trim. It was like I was against a wall. Just more rpms but no acceleration or mph. I've never felt anything like that in a boat. I'll keep trying all the props I can get my hands on. I'll try to post some pics and or videos after my next test session. Thanks!

**David - WI** · 06-01-2021, 09:28 AM

That big diameter gearcase will hurt more, the faster you go.

These guys hit 41 mph at 6k. https://www.boatingmag.com/mercury-115-pro-xs/

**XstreamVking** · 06-01-2021, 10:06 AM

Is this 4'' tunnel vented?

**mrichartz** · 06-01-2021, 10:23 AM

First question.
You said you got the prop cheap what condition were all the props in? I know no one wants to spend money on props they aren't going to use. Props in somewhat ok shape will work down low but as soon as you start going faster they lose it..

**CodeRed225** · 06-01-2021, 12:51 PM

Originally Posted by David - WI

That big diameter gearcase will hurt more, the faster you go.

These guys hit 41 mph at 6k. https://www.boatingmag.com/mercury-115-pro-xs/

Do you mean thattop speed will suffer with the 2.38 ratio?

**CodeRed225** · 06-01-2021, 12:52 PM

Originally Posted by XstreamVking

Is this 4'' tunnel vented?

Explain what this would be. I doubt it is.

**CodeRed225** · 06-01-2021, 12:57 PM

Originally Posted by mrichartz

First question.
You said you got the prop cheap what condition were all the props in? I know no one wants to spend money on props they aren't going to use. Props in somewhat ok shape will work down low but as soon as you start going faster they lose it..

The 19p aluminum was slightly dinged up and i think a little bent. I paid 20 bucks for it but wouldn't have paid 5 if I wasn't looking for something to try. The two 24p props are nice. Probably 8-9 of a 10.

**XstreamVking** · 06-01-2021, 01:16 PM

some info https://www.google.com/search?q=tunn...hrome&ie=UTF-8

**CodeRed225** · 06-01-2021, 02:58 PM

That is interesting about the tunnel vent. Mine is not but I'm thinking I might want to. It makes sense. I wonder how much effect on the prop that will have. I have to do some more research. Thanks for the link!

**David - WI** · 06-01-2021, 07:22 PM

Originally Posted by CodeRed225

Do you mean thattop speed will suffer with the 2.38 ratio?

It looks like the 2.38 gear ratio only came in the large 4.9" diameter gearcase... the smaller diameter gearcase is faster (according to the article linked to).

**CodeRed225** · 06-01-2021, 07:31 PM

Okay, I'm definitely going to vent the tunnel. That has to make a big difference. I just need to figure out where to put it and what size it should be.

I got a hold of Brett at BBlades. Super nice guy, I couldn't believe how much time he spent on the phone with me. So with the 24p props I am pretty much topping out the performance around 5000 rpm. After that I am only gaining rpm by raising and trimming to the point that I am increasing prop slip. Basically I am over propped. The thing I dont quite get is how well it jumped on plane with both 24s. I always thought an over propped boat takes longer to plane. Maybe it has something to do with the 2.38 gearcase and larger diameter prop? I found a 21 lazer 2 from a guy, so I'm anxious to pick that up and try it.

Unfortunately I think my 50 mph goal might be unattainable with the current power. There might be a 115 ProXS for sale soon. A 150 would look good hanging on the back.

**Whaaaaat** · 06-02-2021, 06:55 AM

50 is probably impossible. The prop shop gave you solid advice. I wouldn't vent the tunnel until you get a good solid baseline as is. If you change to many things at once you don't really know what is helping. It hops on plane fast because it's flat bottom, it's there nature. I think that 21 will also be too stiff. 19p I think will be the sweet spot. Flat bottom boats like a deeply buried prop. Other than shallow draft needs I don't think you'll see much gain from motor height, just more slip as bblades stated.