need for speed

[uny1965]

hi all, i'm after a bit more power from my 18hp tohatsu 2 stroke, to tow toys etc.

I have been told by my local mechanic that it is possible, by tweeking the carb etc to upgrade the motor to nearly 25hp, but he reckons that i'll use upto 10% more fuel.

The cost to do this including the service would be £75.

The other option would be to change the prop to a different pitch.
We use our 3.8m inflatable for fishing and towing ringos and was wondering what you all think would be my best option. (do i go for a higher or lower pitch, stainless or aluminium).

thanks in advance

 

[mil1194]

Well......10% extra fuel, for an increase in power of nearly 40% seems to be fantastic.........

 

[PaulGooch]

Well......10% extra fuel, for an increase in power of nearly 40% seems to be fantastic.........

I thought so too, if i could get that for £75, including a service, i'd have someones arm off in a breath.

 

[mil1194]

I'll ask my man if he can do that with my AQ130's. That'd be an increase from 250 to 361. £150 you say.....(rubs chin thoughtfully........)

 

[Wiggo]

Can he do diesels as well?

Seriously, if you need more grunt for pulling toys then more power is needed, not a different prop. A lot of 2 strokes can be upgraded this way, and £75 sounds like a bargain even if it will use way more than 10% more juice. To be honest, even going up to 25hp will be a bit marginal on a 3.8m inflatable (I assume it's a RIB, BTW).

Fuel-wise, I seem to remember an old figure of half a pint per horsepower per hour at full chat for 2 strokes, so 25hp would use 1.5 gallons per hour. Does that sound reasonable?

 

[DAKA]

Can he do diesels as well?

Seriously, if you need more grunt for pulling toys then more power is needed, not a different prop.

Mines just been serviced but an extra 40% power @ £75 please add me to the list too

My 9.8 hp pulls kids on toys but not an adult, 18hp should be enough to tow one adult as long as the engine is able to rev to max , a prop change may be enough.

 

[Davy_S]

Pis*ing against the wind is what you are doing, simply change the motor for a bigger one! you know it makes sense, if it was so easy everyone would do it.

 

[DAKA]

just remembered !

My mate pulled me on the above toy using his 15 hp, 3.4m rib.

no prop change but he did need to turn, which allowed the engine to rev due to reduced load on tow, once up to planing speed it was fine.

 

[Wiggo]

Pis*ing against the wind is what you are doing, simply change the motor for a bigger one! you know it makes sense, if it was so easy everyone would do it.

There's a good chance that a 20 and a 25hp motor are actually the same unit but with different carbs or jets, so the upgrade is feasible. I'm just not sure 25hp is going to be enough for what he wants to do. We have 15hp on a 3m RIB and now that the kids are teenagers it's a bit borderline with a ringo, to be honest.

The other question is what is the boat rated for?

 

[uny1965]

hi all, just to shed some extra light.
the inflatable is a bombard aerotec 380 with the high pressure floor. (rated to 25hp)
dont get me wrong............ we can pull ringos with 1 person on, and 2 up in the boat.
just being greedy for that extra couple of knots.

gps'd - 2 up on fairly flat water at 21/22mph.
seems to perform better with a bit of chop and swell.
would love 25hp, but dont think my old back would take the strain.

this was 2 weeks ago in wales
http://www.youtube.com/watch?v=oGEDpa_lGjc

 

[Davy_S]

There's a good chance that a 20 and a 25hp motor are actually the same unit but with different carbs or jets,

Whilst I would agree in principle with smaller engines ie up to 10 hp, I think you will find it very different with larger engines. Example, the Honda 25hp and 30hp are exactly the same engine, the difference is not simply changing jets and altering throttle stops, both manifolds are different (as well as jets)
Most yamahas are the same, lets face it, if it was so simple, peeps would buy a smaller engine plus a handfull of bigger jets. To me what the OP is trying to achieve seems pointless, (each to their own)