Outboard Power for Spitfire Hydroplane

[JDusza]

We have built an 8-foot Mini-Most hydroplane and we power it with an Evinrude 15 hp. With the stock motor and transom and the wind at our back, we peak at 30 mph via gps. Implementing a jack plate would be a hoot!

We want to build another hydroplane and we have a line on a Mercury 18 hp with helm controls. There is no way the Mini-Most will accept this kind of weight, so we are looking for a slightly larger alternative. We think the Spitfire, advertised at Svenson?s Free Plans, at 10 feet with 51? beam, this may be the right hull. The weight will be about 110 pounds without rigging. The engine must weigh another 100 pounds.

In the 1950?s, the Spitfire was advertised to run 50 mph with a Mercury Hurricane and Quickie lower unit. I have no idea how much a Hurricane weighs.

We are looking for opinions how the weight of the 18 may work on a Spitfire.

Thanks.
J

 

[Bob_VT]

Re: Outboard Power for Spitfire Hydroplane

You need a mercury 25 xs with the racing gearbox/lower unit then you are talking about 75 mph

Here is an older 4 cylinder Merc Mark 30 I ran

 

[JDusza]

Re: Outboard Power for Spitfire Hydroplane

This is very nice, Bob_VT! Well done!
What do you think this 4-cylinder weights? Must be 100 pounds, no? The 18 we're looking at is a 2 cylinder, but, they're big.
We're more in tune with the shallow v-hull for ripply water. How's this 3-point take the corners?
Thanks for your input and the pics!
J

 

[The_Kid]

Re: Outboard Power for Spitfire Hydroplane

In the 1950?s, the Spitfire was advertised to run 50 mph with a Mercury Hurricane and Quickie lower unit. I have no idea how much a Hurricane weighs.

My Hurricane Super 10 weighs around 65 lbs. I know that when I had it on a 12 foot john boat it moved along quite nicely. It sounded like someone was chasing you with a chainsaw. It's been sitting in the shed for about 15 years now.

 

[Bob_VT]

Re: Outboard Power for Spitfire Hydroplane

This is very nice, Bob_VT! Well done!
What do you think this 4-cylinder weights? Must be 100 pounds, no? The 18 we're looking at is a 2 cylinder, but, they're big.
We're more in tune with the shallow v-hull for ripply water. How's this 3-point take the corners?
Thanks for your input and the pics!
J

It was closer to 140 lbs and the 3 point cornered fairly good...... However It did get "stuffed" a few times which is abrupt!

 

[Chinewalker]

Re: Outboard Power for Spitfire Hydroplane

Hey Bob - that boat looks suspiciously like a Sid-Son. I've got an earlier Sid-Craft C/D hydro on the barn. Runs about 62 with my 30H...

J - the 10-footer will handle the weight of your 18 no problem, but you will not get anywhere near 50mph with a stock service lower unit. The older Mercs with the Quicksilver lowers run a square gear ratio and a surfacing gearcase & propeller, along with open exhaust. Even the Super 10 of The Kid's will perform well, as it has a small gearcase and 16:21 gear ratio (same ratio as that used on what is called a Mercury Mark 20H Conversion). Some guys use the Super 10 style lower unit as a starting point to build up a Rat Racer using a later 22-cube 20hp powerhead.

Also, as was noted by Bob, if you put the 18hp powerhead on a 25XS lower, you'll see some REAL speed.

 

[JDusza]

Re: Outboard Power for Spitfire Hydroplane

Bob_VT, Abrupt. That is a word to use.
The_Kid, Sweet Hurricane 10. Thanks for the pics. You have a gem.
Chinewalker, 60+ with 30 hp. he he he he he ....... Gotcha on speed. There's only so much from stock. I sort of expect to see mid- maybe upper-30s mph.
Thanks for the input, guys. I think I'm gonna see a buddy about a Merc.....
J