How do the old 15a type outboards get over 50 mph?

G’day,

I’ve just built my first boat, A glen l minimaxxed which is 8ft long and 4ft wide. Basically 3 sheets of 6mm plywood cobbled together with a 15 hp motor clamped to it. Which is a 1997 mercury 15hp 2 stroke I borrowed off my fishing boat.

Took it out to Copeton Dam in North NSW for a run and had a bloke on a jet ski drive along beside me to see how fast she goes and he says i’m doing 50 km/hr.

I have read about the old 15hp racing outboards getting 50 or more mph and this has me scratching my head how they get those speeds. I’m running a 9.25x12 pitch solas prop 8 spline and dont have a tacho but by ear it sounds about 6,000 or so. Tried the original 9x9 and it was clearly over revving.

I’ve never opened up an outboard motor to see inside one other than taking the gearbox off to change a water pump so other than that im a outboard noob. I have mucked about with 2 stroke bikes changing port timing and comp and managed to get an old 2 tonne dual cab 4x4 farm hilux down the 1/8 mile in 8 seconds with a home made turbo setup on a holden v8 I’d welded in there years ago so I can handle a spanner and a die grinder a bit. I’m keen to learn how a bloke might do a bit of modifying on a merc 15 to get it going faster than 50km/hr like she’s running now.

Can any of you fellas steer me to some info or books or first hand ideas that can get more power out of this old merc 15 ? Looking at the motor I cant even see how the heads come off.... looks like barrel and head is 1 piece not sure. I’m not sure how to post pictures here either but theres 1 of the boat in my profile page.

Thanks

The Merc 15 cu in engines (KG4H) that were used to get 50+ had special streamlined quicksilver gear cases with 1:1 ratio and only forward gears and short exhaust mid sections, your service engine does not have these and probably a 1.75:1 or 2:1(?)or so gear ratio. The exhaust mids were shorter from the transom clamps to the prop shaft center so they could run higher on the transom with special small high pitch bronze or stainless steel props that were run with about 1/2 the prop out if the water for less gear case drag and higher revs in the 7000 rpm range. Forget modding that Merc 15 engine the Quicksilver gear cases and tower and props is the way and they do not bolt up to your Merc 15 far as I know.

Back in the race days my 10ft hydro would run the mid 50's with the Merc KG4H (15 cu in) race engine and the mid 60's with the Merc 20H (20 cu in) or Merc 25SS (22 cu in) with good tune, right engine set up and "props". The 20H and 25SS had 1:1 and 16:21 gear ratio options most used 16:21 ratio. Avatar is with the 25ss back in the 70's at a APBA race in south Fla.

What year is your Merc 15?

My merc is stamped 1997,15m or short leg. (40cm motor board) max 6000 rpm with serial number starting with OG4612.
8 spline on prop with 2:1 gearcase.

You refered to them as 15cubes, rather than 15hp like mine says. Did the older ones have more than 15 hp or about the same?

Great info thanks.

For a 1997 15 hp I thought is was a 15 ci in displacement, I thought the old ones were the 1950's era 15 cu in engines. Your 1997 engine is 2.38" bore x 1.8" stroke and the total displacement is 16 cu in.

Per my old Mercury Shop Manual:
The 1950 - 1952 KG4H 15 cu in was rated at 7.7+ hp;
the 1950 -1952 20 cu in KG7H was rated at 10+ hp ;
the 1954 -55 20 cu in Mark 20H was rated at 16+ ;
the 1956 Mark 20H 20 cu in was rated at 20+ hp ;

and all hp was rpm dependent and when run at 7000+ rpm they would pull more hp than noted. Mr Carl Kieikheafer Mercury factory owner listed under rated hp for his engines, sales reasons for his competition not to know all. All these had the Quicksilver gear case and the corresponding short exhaust tower for running the engine high on the transom. That was the H designation in the name.

These engines were set up on hydros to have the center of the prop shaft to be near the bottom of the boat so about 1/2 the prop is above the water line. Your engine does not have such a short dimension to allow you to run the prop shaft center that high with a 40cm (15.75" ) transom you note.

Hello Chad Morgan,

In addition to the above reasons that you can not get 50 mph or so, your Glen L Mini Maxed is not a three point hydroplane.... a three point hydroplane creates an air tunnel under the boat to lift it out of the water creating less boat drag. A three point hydroplane literally flies over the top of the water with the proper set up and propeller.

Of course with more speed comes more risk.... it takes a learning curve to run a three point hydroplane at speed. If you would like to build a ‘real racing three point hydroplane’ here is a choice: Sorensen Woodcraft would be a recommendation..... Darrell Sorensen has full size pattern plans. Try sorensenwoodcraft.com.......phone: 559-665-2690..... he is in California, USA.

Good luck on your project. Regards From, Dean Hobart, racing boats since 1962.

This little Minimaxxed is a bit strange to drive compared to my 3.8 meter aluminium fishing boat. She stands up and points at the sky on take off then goes bow down at full throttle as she chops the waves off and they smack you in the face and try to blow you out the back untill I take on enough water onboard that I need to bail it out and go at it again. 3 air tight compartments keep her floating positively above water level whilst swamped so thats handy.

Then in the turns I gotta lean right in and lay on the side of boat as she slides sideways through the turns. It’s a hairy ride as the outside hits holes and waves and bites in and tries to highside you out of the seat Hahaha. Its damn good fun but I dont think I’d trust it at 50mph and I’m certain I would roll it in a race pushing it for the checkered flag.

I have been looking around for plans for a faster race type boat and have 4 different plans in the mail right now coming for me to look at and see which I will build next. Ive got 2 coming from a guy called Don from somewhere in USA which look like some sort of small hydroplanes. 1 is for a yamato 331 and the other for a 15a type motor. The other 2 plans I’m not sure exactly what they are but ones a tunnel boat and the other may be as well or it could be a mono hull. Wont know untill they arrive this arvo or next week and they are from an old boat racing guy called Ralph D and took a bit of work to get my hands on and they are for the yamato 331 motor.

Thats part of the reason I’m trying to find out if I could mod my merc15 either to use it on my minimaxxed or maybe one of these other designs I will build. Otherwise I can put the merc back on my fishing boat and grab a yamato and a new outboard that might have the possibility of being raced. I would need to see which classes and rules along with boat designs I could use here in Australia before I start the builds.

Looks like I’ve got a lot to learn but looking forward to jumping in and having a go.

Just had a look at Sorrenson’s facebook page and dillons racing and there are some nice looking boats on those blokes websites. I better wait and see what the plans I’ve already got coming look like and try to find a rule book before I go ordering any more plans. I did like seeing Clints red tunnel boat videos as he stomped it around the track in those few races he posted in his build thread here. It was definitely standing out from the others. My problem is not knowing which designs are able to enter which classes here and which ones are faster or better handling than the others. They all look like they could win with the right guy driving them. I’m a complete rookie in that regard having never been in a boat race..... yet! :o

Mailman just dropped off a tube of plans. Couldn’t wait to open it..... Inside is 1 photograph of the completed tunnel boat and 2 large pieces of paper with a bunch of 1:12 scale drawings and hand written notes and squiggles and measurements from the old school legend himself, Ralph De silva !! To build this tunnel boat he calls “ THE COMET”. I could’t find any other info about this design online but rumour has it that Ralf designed and built this boat for his mate to race who also wrote a part in Ralphs book. Nice piece of boat racing history and I would be honoured to build and drive this boat..... as soon as I learn how to read and loft these type of plans. Hmmm.

Adding a fin to the hull will help immensely ....Reguarding placement,do the plans specify distance from transom to fin ??

Yee haa !!