how fast could it go with a larger engine?

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redlinj

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My '88 Sylvan 21' super sportster, (same as Starcraft SS 21?) came with an 88 hp Johnson ob. This motor pushes the boat about 33-34 mph as shown on the GPS. Is there a formula or general rule of thumb to find out how much faster a larger motor could make this boat go? It is rated for 200 hp. It is primarily a fishing boat for me, but when Lake Erie is flat as a pancake I kinda like to open it up! Thanks!
 
Watermann

Watermann

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Re: how fast could it go with a larger engine?

The only formula I know of for going big on OB's is the fuel consumption rate... 1 gallon an hour for every 10 HP at WOT. Easy to do that math when stopping at the premium pump.
 
alldodge

alldodge

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Re: how fast could it go with a larger engine?

redlinj said:
My '88 Sylvan 21' super sportster, (same as Starcraft SS 21?) came with an 88 hp Johnson ob. This motor pushes the boat about 33-34 mph as shown on the GPS. Is there a formula or general rule of thumb to find out how much faster a larger motor could make this boat go? It is rated for 200 hp. It is primarily a fishing boat for me, but when Lake Erie is flat as a pancake I kinda like to open it up! Thanks!
Click to expand...

The guy that would know is Walt T. PM him and send with the link to this thread and I'm sure he will advise
 
jbcurt00

jbcurt00

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Re: how fast could it go with a larger engine?

redlinj said:
My '88 Sylvan 21' super sportster........ Is there a formula or general rule of thumb to find out how much faster a larger motor could make this boat go?
Click to expand...

I figure roughly 3mph slower then a Starcraft SS 21 :laugh:

I suspect a 200 would push it along fast enough at WOT that a majority of the boating public wouldn't want to be aboard.

How's your back & kidneys?
 
barato2

barato2

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Re: how fast could it go with a larger engine?

i'm a little dubious about the Crouch speed calculator. assuming hull constant of 180 and a loaded weight of 1500lbs, it shows my 18' holiner hitting 45+mph with the 100hp dorm fridge......i suspect from others' results with this hull that this is about 10mph optimistic
 
ctswf

ctswf

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Re: how fast could it go with a larger engine?

I usually dig deep on google and find people with a similar setup when I have this question. Something like "21 starcraft 200hp". I only say starcraft because you usually get more to compare

Anyway this guy claims 68mph, with 21' tin and 200hp. that sounds high to me...and crazy :)
1979 200hp v6 evinrude on a 22' 1979 starcraft supersport v5 - YouTube
 
barato2

barato2

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Re: how fast could it go with a larger engine?

with special attention to the transom? ;)
 
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