Starting from scratch Oh boy

Tail_Gunner

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Not to much experience here with this type of setup, 78 Glastron GT 150 675lbs dry weight add 400lbs total load 1000 lbs.
1977 Johnson 70hp 2 stroke 120 lbs compression on all three cylinders runs very strong. Turns a older 3 blade right to 6000.
I believe it a 2.25 drive ratio.

So I use turning points prop calc and it comes up with a 21, using a prop slip calc that gets 48mph@9% slip. That seems just a smidge high to me for some unknown reason. Any input here or experience would be appreciated
I will be using a 4blade aluminium probably Solas top end is of no value to me.
 

ahicks

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Knowing what size this was would provide invaluable info...

"Turns a older 3 blade right to 6000."

In the most basic runabout/ski boat terms, 21+ pitch props are for sale everywhere because few setups will allow their use. 15" - 19" are hard to find, generally because they are the most popular. If I need to guess, it's likely going to be a prop in that range.
 

Tail_Gunner

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Knowing what size this was would provide invaluable info...

"Turns a older 3 blade right to 6000."

In the most basic runabout/ski boat terms, 21+ pitch props are for sale everywhere because few setups will allow their use. 15" - 19" are hard to find, generally because they are the most popular. If I need to guess, it's likely going to be a prop in that range.

17p that seemed right to me.....but she pulled hard the last 1000 rpm then just ran out of air. I just am reluctant to plop another hundred on a 20/21.
 

ahicks

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To make sure I'm following. You're saying your engine pulled 6000rpm with a 17 pitch? And your considering a 20-21" pitch? If that's the case, I think you're going to be pretty disappointed with the 21".

What is your target max rpm?
 

Tail_Gunner

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To make sure I'm following. You're saying your engine pulled 6000rpm with a 17 pitch? And your considering a 20-21" pitch? If that's the case, I think you're going to be pretty disappointed with the 21".

What is your target max rpm?

6000 for this outboard, after a bit of lurking ive found this boat commonly runs with 18/21 inches of pitch with 70/110 go engines. It can also achieve 49/55 mph speeds safely. Lol it took me by surprise actually both prop size and speed, I though of a name Buzzcicle
 

NYBo

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I find it hard to believe a 70 can spin a 21" prop to 6000 RPM on a boat that size.
 

ahicks

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Target rpm is 6K which you are already turning with a 17p, and your plans are to go with something in the 18-21" range to increase performance? Let us know how that works out for you.... Especially when using a 4 blade prop, with no priority regarding speed.
 

jimmbo

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The WOT range for that engine is 4500 - 5500. It is not going to make more power at 6000, it will in fact be making less and will deliver a lower top speed. Gear ratio is 29:12 which would be 2.41:1. A 17, possibly a 19, would be about the correct choice for that boat, Does it have power trim? Is your tach accurate. and set correctly?
 

jimmbo

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Agreed i would be stunned, smiles a 70 /110 hp....i can see a 100 perhaps getting there however

Even a 99 in. 140 would be very hard pressed to get that RPM with a 21 on that hull, it might get to 5300-5400
 

flyingscott

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I know people who have owned those boats and speeds with an 85 hp was the high 40s. I would check your tach again and GPS your speed.
 

Tail_Gunner

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The WOT range for that engine is 4500 - 5500. It is not going to make more power at 6000, it will in fact be making less and will deliver a lower top speed. Gear ratio is 29:12 which would be 2.41:1. A 17, possibly a 19, would be about the correct choice for that boat, Does it have power trim? Is your tach accurate. and set correctly?

Is that gearing you posted accurate,by the book so to speak. I'm using a cell phone alot lately and can't access the boat library

I guess I should add a few things to this scenario, starting from scratch period. The engine is set far to low the cav plate is actually 1to 2 inches under the hull the engine spun the prop right to 6000 its set up very badly and will be tinkering with things next week...aka starting from scratch i was just taken back by prop and slip calculator so I started posting to get some insight on this small boat motor combo. Trying to make a educated WAG
Lol there's lots to this restoration and a $100 is a $100 need to keep close track of them
 
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jimmbo

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well the 70 hp 49.7 3 cylinder looper had this ratio for many years
img638a.jpg
 

Tail_Gunner

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Thank you Jimmbo that pushes things to a 21 on paper just when I had decided a 19 would be a good guess. I have little experience with small boats and outboards but a 21 sounds well wisho meter comes to mind. So I think im going to the marine boneyard and find a 20 21 lift the engine get a good tach and see what happens.
 

jimmbo

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Make sure your tach is accurate and set correctly, I say a 21 is too much prop for your boat, and still stand by my recommendation of a 17 possibly a 19.
 

ahicks

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I'm not sure why he's asking Jimmbo. It sounds like his mind is pretty well made up and he's just trolling for somebody to agree with him.
 
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