AeroMarine Sprint Cat 21

[RCUK John]

Anyone sucessfully running an AeroMarine Sprint Cat 21?

If so, can I have details of the prop depth, the prop diameter/maker and distance from the transom?

I am running a CMB 21RS on a 180º manifold with a Valvola tuned pipe on 10%nitro.
So far all that I have is a lot of "thrashing & splashing" but no significant speed.

If I put the boat into a turn and the engine seems to go onto "pipe" but as soon as it is back on a straight line it is back to "proceeding" around the water.
Tried different props, prop depths and different tuned lengths but so far to no avail - this is the first one to beat me!

John

[Ron Olson]

First, get rid of the 10% and put some higher nitro fuel in there. Small engines like the .21 thrive on high nitro. I know that nitro isn't cheap there in the UK but a hot engine like this one needs the stuff.
What seems to be hapening in the corners is that the Cat is leaning just enough to raise the strut/prop a little higher out of the water freeing up the prop. Raise the strut just a little higher but you don't want it so high that it cavitates in the corners.
What prop(s) are you using?

I've only had a little experience with this boat, rebuilding one for a friend.

[RCUK John]

Thanks Ron
Nor is Gas at £1.05 a litre - That is $9 a UK gallon.
The strut is as high as it can go, it is touching the top of the tunnel.
To raise it further means removing the flexidrive and poking it out at the bottom of the transom so going full surface drive instead of semi submerged.
I have run sucessfully on 10% for years, my CMB 90 in a Challenger 48 most certainly is on the "quick side of fast".
Any info on prop diameters?

[Ron Olson]

The CMB's can turn some big props but you might find that you may have to go in the opposite direction with them. Seeing how CMB used to make the MAC engines, they use small props like the Octura X-437/3 to get them to wind up. The tuned pipe length should he in the neighborhood of 8-1/2" from the glow plug to the fat part of the pipe.

As an aside, a lot of nitro engines on your side of the Big Pond are made for low nitro use where on this side they are made for high nitro use as the manufacturers do understand that the price of nitro is way up there.

[RCUK John]

As an aside, a lot of nitro engines on your side of the Big Pond are made for low nitro use where on this side they are made for high nitro use as the manufacturers do understand that the price of nitro is way up there.

Thanks Ron for your time.
My pipe is at 8½" and I have been trying from 38 to 42 dia 2 blade props.
10% is pretty standard over here for most of the circuit type boats. The Multi boats (FSRV) race boats seem to use higher %age.
10% nitro fuel is about $30 for a (UK) gallon.

[Ron Olson]

Wow, I've been paying a hair over $20.00 a gallon for 65%! I heard that the price is climbing up again this year but I'm set for a while. Going gas will help with keeping the fuel cost down for me.

I hate getting beat but I seem to be a glutton for punishment. :)
I can see your frustration as the centerline of the strut should be just slightly above the bottom of the sponsons and level with them. You've got enough experience with other boats so I can see why this one is driving you nuts. You might find out that the problem lies somewhere else, a place that you wouldn't expect it to be.

[RCUK John]

You might find out that the problem lies somewhere else, a place that you wouldn't expect it to be.

That place could be on Ebay if this boat soon does not get going.:mad: