Help with motor selection

[nate22]

Hi everyone,

First I would like to thank all the contributors of this form, I have learned a lot from reading the many post on here. Thank you

I'm planning to race in my local offshore circuit next year and need help selecting an engine.

The class I will be racing in gives you two options, stock 1.3liter 90hp (transom mounted water pickup allowed) or modified 850liter 70hp engines (no nitrous and exhaust stacks allowed).

The hull that I will be using is an Ocke Mannerfelt 19ft stepped V bottom hull that weights around 450lb. http://www.mdteam.se/boats/B19.asp?s=3&type=race

Boat transom is 15in

I'm thinking about using a sst60 motor, moded Stinger 75 engine, or moded Yamaha 70hp motor.

How do you think these engines would stand up to a stock 90hp engine? I'm manly worried about the torque being sufficient on the 70hp engine vs the 90hp for the weight of the hull.

what would you use if you were racing in this class ?

[Mark75H]

That's a real toss up. I think you would have a good chance with either.

Let's hear what others say

[JohnsonM50]

Its an old rivalry.. big 'stock' cubic inches vs smaller more hi performance motors. Tuff call but Id think the modified idea can work if the weight & drag of the hull are compatible. The stock 90 with a pick up on the other hand can be run at a hi performance set up. [In other words I have no idea] but Good Luck.

[nate22]

Lets compare to engines of the same manufacture.

90hp Yamaha
weight 261 lb
20in shaft motor
rpm range 4500-5500rpm

70hp Yamaha
weight 228 lb / 33lb less then Yam 90hp
I can modify it to a 15in shaft motor less weight maybe 10-15lb
rpm range after mods 6,500 to 7,000rpm
I can modify the lower unit for less hydrodynamic drag and use a low water pickup

So if rpm = speed the 70hp should be the whey to go? also I would have -40lb off my transom for better handling.

I'm gassing I can mod the 70 to create 90hp and still be reliable and still have less weight on the hull.

The only thing i'm concerned about is the toque of the 90 vs the 70. Is it better to have more rpms or toque all things being equal to create speed?

The hull is not a deep V. I have been told it uses the same concept as catamarans to trap air to create lift and use less of a running surface.

[JohnsonM50]

Lets compare to engines of the same manufacture.

90hp Yamaha
weight 261 lb
20in shaft motor
rpm range 4500-5500rpm

70hp Yamaha
weight 228 lb / 33lb less then Yam 90hp
I can modify it to a 15in shaft motor less weight maybe 10-15lb
rpm range after mods 6,500 to 7,000rpm
I can modify the lower unit for less hydrodynamic drag and use a low water pickup

So if rpm = speed the 70hp should be the whey to go? also I would have -40lb off my transom for better handling.

I'm gassing I can mod the 70 to create 90hp and still be reliable and still have less weight on the hull.

The only thing i'm concerned about is the toque of the 90 vs the 70. Is it better to have more rpms or toque all things being equal to create speed?

The hull is not a deep V. I have been told it uses the same concept as catamarans to trap air to create lift and use less of a running surface.

It may come down to what props you can spin [or not]. The more hi perf, the less range of what you can use. The 90 will have more options but doesn't mean better ones The minus 40 lbs is attractive especially if you need to be nimble on course. No getting round that torque tho, still a tuff call.

[nate22]

Some very good points made thus far.

I have talked to a couple of performance tuner shops, and they all think the modified 70hp can work and should perform better, because of the more broader rpm range once modified. One particular shop suggested that I use a 56 c.i power head vs the 49 c.i one. Said the added displacement was much better then the 49 c.i head, so I guess that eliminates the 70hp Yamaha and 49 c.i OMC out of the equation.

I guess my only question now is reliability. How reliable is a high compression 70hp outboard engine turning around 7000 to 7,500 rpm jacket up to about 100+hp? How many hours are you racers getting before doing a rebuild on the engines you run.

I have about 15 race days per season to complete.

keep you opinions coming

[fs5]

hi mate ,there was a couple of guys on here that mess'd with the 70 and 90hp yam's on slt's.i'm not sure if it was fastfred or outboardsunlimeted/berg .but they used to run modded 70hp yam's and one of them shortend a 90 and bolted that on !!! i'm sure they'd let you know which one was faster .

[Karl Kretzmann]

Hi Nate22,

I ran in the world champs and Euro Champs in Class 3 1,3L offshore. We an an Argocat 17 with 90 hp Yamaha. Incredible rig!
we ran hydraulic jack plates and low water pickups ( motor mounted). we could only blueprint motor to factory spec with no mods allowed. we ran 26" and 28" cleaver props.

If cost and reliability are an issue then run the stock 90 option and turn bigger props!

The "Bat" boats are a little to heavy for the 70 in my opinion.

[nate22]

Hi Nate22,

I ran in the world champs and Euro Champs in Class 3 1,3L offshore. We an an Argocat 17 with 90 hp Yamaha. Incredible rig!
we ran hydraulic jack plates and low water pickups ( motor mounted). we could only blueprint motor to factory spec with no mods allowed. we ran 26" and 28" cleaver props.

If cost and reliability are an issue then run the stock 90 option and turn bigger props!

The "Bat" boats are a little to heavy for the 70 in my opinion.

Hi Karl Kretzmann,

Yes the Argocat is a very incredible rig, I understand they are very fast with a 1.3liter outboard.

How fast did your one go with the setup you had? How about handling? I wanted to get an Argocat but was told they don't handle good in the ruff stuff.

How did the Batboats perform like that you were competing against?

Were they able to match your top speed?

Did your boat have to conform to the UIM ruling? I have read in the rules that you are allowed to cut the rev limiter, advance the timing? (did you do this to your engine) and add a low-water pickup to the engine. I'm guessing the only way you can get good speed out of these 1.3liter boats is to jack the engine way up to run a surface-piercing prop, and to let the engine rev completely out.

This was why I thought a modified 70 would be the way to go for top speed. In the chop was my main concern with the lack of torque that a 70hp has to a 90hp. I think you could be right being they designed the bat-boat to be used with a 90hp engine.

fs5 that would be nice to have some opinions from those that have ran a 70, and 90hp engine.

thanks guys

[Karl Kretzmann]

Hi Karl Kretzmann,

Yes the Argocat is a very incredible rig, I understand they are very fast with a 1.3liter outboard.

How fast did your one go with the setup you had? How about handling? I wanted to get an Argocat but was told they don't handle good in the ruff stuff.

How did the Batboats perform like that you were competing against?

Were they able to match your top speed?

Did your boat have to conform to the UIM ruling? I have read in the rules that you are allowed to cut the rev limiter, advance the timing? (did you do this to your engine) and add a low-water pickup to the engine. I'm guessing the only way you can get good speed out of these 1.3liter boats is to jack the engine way up to run a surface-piercing prop, and to let the engine rev completely out.

This was why I thought a modified 70 would be the way to go for top speed. In the chop was my main concern with the lack of torque that a 70hp has to a 90hp. I think you could be right being they designed the bat-boat to be used with a 90hp engine.

fs5 that would be nice to have some opinions from those that have ran a 70, and 90hp engine.

thanks guys

Hi Nate,

We used to run in the late 70's and early 80 mph.
in real flat conditions the bat boats never stood a chance and in chop (1 to 2m) will were still quicker. in real rough stuff ( swells 4 m plus) the bat boats had an advantage, but not big enough to completely dominate races. normally there were areas on the circuit that were flat enough for us to make up the lost time!

we did conform to strict UIM rules. they checked after every race! being WOC and EOC events.
The real secret to the speeds for low water pickups, vertical jack plates and 26/28" surface props!! we ran the boat real "loose" to get those speeds, but the argo was amazingly controllable when run loose! We ran at around 7800 rpm.

A friend we met in Finland, ran the argocat 15 with a 70c and all i can say is WOW! that little thing was quick!

I'm sure that if you ran a 70C on the bat, it could work,but there wouldn't be much power/speed at the bottom end. you would need some length to wind it up!