Need for speed

[Keithhutchinson444]

I have a 10 foot hydro. With a 25hp johnson from 83. It goes 40km. It has a 9.5x 12 inch prop. So. If 15hp can hit 100 km how can i get more speed out of this little guy. Ill never be racing it, its only for my fun. Help me out here guys.

[ZUL8TR]

Need answers:

1 How much does the rig weigh when running at 40kph?
2 You have a standard service engine F-N-R I suppose?
3 What is the present set up of engine on transom, i.e. how deep is the prop shaft below the rear bottom? What angle of trim on the prop shaft is set?
4 What rpm are you running?
5 Post Pics of the hydro and engine?
6 What is the condition of the engine? Engine condition and tune specifics are a whole other category. The engine has to be mechanically right and tuned well to turn the prop at the rpms.


You can get an idea of the speed you should be near with: pitch, engine rpm and gear ratio of lower unit.

http://propellerhub.com/boat-speed-calculator.html

Just use the smallest slip available as an idea of a theoretical max.
Slip is always there we try to make it small with right prop and set up, etc.
Remember this is just a guide systematic testing reveals what is needed

[Keithhutchinson444]

It's running weight I don't know. I think it's 140lbs. It's fiberglass 10x5 and the motor
Is 120lbs or so
It's standard gearing
The trim I've played with getting it from 38km to 40. The height I haven't played with yet. I'm making a cavatation plate to make that process easier for me
The tac isn't hooked up yet. Still learning as I go. It's on the boat with the wires I just need to wire it all in.
The motor is getting a once over this winter. Carb clean, gear oil, plugs impeller and rings
Thanks for your interest , it's really hard for me to find people into this kinda thing. I'm the only guy in the city with one of these little things

[Keithhutchinson444]

I'd be very curious to know my gear ratio. And cc of the engine It's kinda trick to find info on a 83 j25TECTE

[ZUL8TR]

Easy to get the gear ratio. Service engines are geared so the prop turns less than the engine each revolution. Mark your prop position relative to a fixed point on the gear case, rotate the flywheel 1 rev (360 degrees) and note where your mark is relative to where it started. If you can determine the portion of a full circle the prop turned the gear ratio is calculated as:

360 divided by the degrees prop turned.

Your engine is probably in the 2:1 ratio range.

Can also look it up in a spec manual.

[DeanFHobart]

I have a 10 foot hydro. With a 25hp johnson from 83. It goes 40km. It has a 9.5x 12 inch prop. So. If 15hp can hit 100 km how can i get more speed out of this little guy. Ill never be racing it, its only for my fun. Help me out here guys.

Hello KH 444,

So to answer your question of how to get more speed from your equipment, you first need to know how a 15 HP motor can go 100 kph (62+ mph). The people doing this are experienced boat racers. Usually a special designed boat specifically for straightaway runs. The motor is an "A", OMC motor with a racing gear case with a very special propeller for going in a straight line only. All of this equipment is highly tuned and set-up to the max. With lots and lots of testing.

Your speed of 40 kph (25mph) with a "Standard 25 with a "Non-Racing gear case" is probably reasonable. You could most likely increase your speed by "Jacking Up" the motor on the transom and different propellers. But I wouldn't recommend it. Propellers can be very expensive. Also, we do not know what kind of boat you have.

However, most of all, have fun...... And, Good Luck. Be sure to wear a helmet and life jacket..... with a Kill Switch on the motor.......... Like they use on Jet Skies. When you go out of the boat a tether to the Kill Switch shuts off the motor.

Best Regards...................................

[Keithhutchinson444]

How do I get ahold of these A gear boxes? Could I get a buddy machinist to make the gears?

[ZUL8TR]

Don't waste your time and $$ for the A gear case for that engine. Just use the engine you have, raise it up and spend the money on a prop to get the revs right and speed up and have safe fun. Contact Ron Hill.

[Fastjack]

Ron Hill Propellers ( prop 19 on Ebay ) has propellers for stock fishing motors and could possibly add 10 mph to your stock fishing motor set ups top speed .

[Keithhutchinson444]

Would all Omc 13 spline props fit my motor? Would a 17inch off a 40 fit my 25 if both are 13 spline?

[ZUL8TR]

Try it for proper fit, but 17 pitch is mighty tall for that 25 to get the revs. But try if it fits, raise up the engine and see what happens.

[Fastjack]

The boat looks like a G W Invader out of Sharpsville IA. I had on in the early 70 's with a Merc 2 cyl - 35 hp - would run 40 mph with a 17 pitch bronze Merc 2 blade prop. Perhaps a upgrade to a 402 Merc would be the best bang for the buck .

[DeanFHobart]

Hello KH 444,

So to answer your question of how to get more speed from your equipment, you first need to know how a 15 HP motor can go 100 kph (62+ mph). The people doing this are experienced boat racers. Usually a special designed boat specifically for straightaway runs. The motor is an "A", OMC motor with a racing gear case with a very special propeller for going in a straight line only. All of this equipment is highly tuned and set-up to the max. With lots and lots of testing.

Your speed of 40 kph (25mph) with a "Standard 25 with a "Non-Racing gear case" is probably reasonable. You could most likely increase your speed by "Jacking Up" the motor on the transom and different propellers. But I wouldn't recommend it. Propellers can be very expensive. Also, we do not know what kind of boat you have.

However, most of all, have fun...... And, Good Luck. Be sure to wear a helmet and life jacket..... with a Kill Switch on the motor.......... Like they use on Jet Skies. When you go out of the boat a tether to the Kill Switch shuts off the motor.

Best Regards...................................

Yo KH444 Again,

I thought you had a Hydro. You do not have a Hydro. 25 mph is most likely all you're going to get..... You will need more Horse Power to go faster.

Good Luck.

[Keithhutchinson444]

How is it not a hydroplane? I know its not a race boat

[ZUL8TR]

Well on this site a hydroplane has sponsons and rams air underneath, you have a runabout type hull. They have more water drag and more power is needed to overcome that. Race boat doesn't matter it's the design of the hull.

[Chinewalker]

Your motor is one of the old OMC 31.8 Cubers (521cc). Solid motors. A few things you can do to up the power - your motor shares much with the 35hp of the same era. As such, you can mix and match some parts to get a bit more horsepower out of it. The 35hp carb and intake manifold would be a good start. If your motor has the through-hub exhaust lower unit (exhaust exits the propeller hub) then you can find some pretty good props as that motor and unit were used on some small tunnel hulls. If your motor has the exhaust exiting the bottom of the AV plate, prop selection will be a bit more limited.
You mentioned adding something to the cavitation plate. Don't. When you raise the motor, it'll be out of the water and will not have any effect on your performance.
If your motor has the through hub unit, it will have 13:28 gears. If it has the AV plate exhaust, it will have 12:21 gears.

[johnsonm50]

I can't see the pic well enough to be sure it's not a 22ci but is probably the 31.8. With the non thru hub its 25 or less hp & mid '70s they were all thru hubs later on. Speed can be obtained but bolt on hp w/ better prop choice is easier no doubt. I've done alot of work with a non thru hub & got 52[gps] on a vintage B hydro & 65 on a semi modern C hydro with a Yamato case adapted. The Invader would not perform lke that of course but is a popular boat & there's probably alot of information available on BRF

[Keithhutchinson444]

Hey there
Thanks for the info, its tricky to find for some reason. Ill skip the cavatation plate then. The exhaust is above the prop.

[DeanFHobart]

Hello KH444,

Does it have sponsons? If it doesn't have sponsons, it is not a hydro.

Again, Have fun...............

[jp183w]

This is a GW Invader. We have one fully restored with a ported Mark 55 on it. You do not want to jack the engines up to far. With a completely
flat bottom the lower unit is doing all of the turning. You should be able to find props for these engines fairly easily on e-bay. Going one size
higher in pitch would be your best choice. This will make the boat harder to get up on plan thou.

[Keithhutchinson444]

If its not a hydroplane is still cool if i hang out with you guys?

[ZUL8TR]

If its not a hydroplane is still cool if i hang out with you guys?

Yes we like to help. I sent you a PM

[DeanFHobart]

If its not a hydroplane is still cool if i hang out with you guys?

Thanks KH444,

It is absolutely cool to hang out here. We needed to know exactly what kind of boat that you have so we can help you further.

Depending on your budget, would you be able to get a motor with more horsepower? Say a Yamaha 40. I think they run a Yamaha 40 in the tunnel boat class called Sports C. On a tunnel boat they go 60 mph or close to your original thought of 100 kph. I'm thinking that motor would get you to 40 - 45 mph or 60+ kph. I know somebody makes Racing Propellers for these motors. See below.

Could some one give the year and model of the Sports C motor that is now being run?

Also, Ron Hill of Hill Marine...... He also has the web site called "BoatRacingFacts.com". He would be able to make you a better propeller for your current motor. He can be called on: 714-608-7147. His company is in the Los Angeles area of California.

Good Luck and Have Fun............................................... ............................

[ctpdsr]

This is a GW Invader. They came in two configurations, one rated for 25hp and one with a beefed up transom rated for 50hp. I have a restored version of the later with a ported Merc44 mounted on a lightweight MK40 service tower and non-shift gearcase. About 55MPH with a bronze Oakland-Johnson prop. A true race motor will not work on this pleasure hull, but a 50hp fishing motor is too much weight. Not a rookie project, but if you can find some help, this is the best possible set up on that boat. Also - the factory steering setup on these hulls is a weak link. Spend some time there including beefing up the dash thickness. Good Luck!