outboard question

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ortogo

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I'm looking at getting a new small outboard for my 9foot fibreglass tender to replace a 35 year old 4hp Johnson. The Johnson measures 17 inches from the mounting bracket to the top of the cavitation plate which exactly matches the dinghies transom. The outboards I've been looking at measure either 15 inches or 20 inches, my question is, is it better to go for the longer or shorter length or is it one of those things that won't make much difference in the real world
 
My opinion in a tender would be to go longer or you will probably cavitate once you get going.. You can always put a spacer in on top of the transom to raise the outboard by a couple of inches if needed.. With one that is too short there isn't much you can do..
 
I'd get the 15 inch. At the D speeds you'll do with 4hp and 9 feet the 2inches too high won't affect performance nor induce cavitation. You may be able to cut 2inches off the transom top edge anyway, if you don't like how it runs.

All that said I'm very surprised at the 17 inches. Back in 1981 the industry was very firmly fixed on 15 and 20 inches, and had been for a long time. You 100% sure on the 17"? It's not 15 and you're measuring it on a diagonal, is it? If so my 15 inch suggestion above makes even more sense
 
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jfm said:
I'd get the 15 inch. At the D speeds you'll do with 4hp and 9 feet the 2inches too high won't affect performance nor induce cavitation. You may be able to cut 2inches off the transom top edge anyway, if you don't like how it runs.

All that said I'm very surprised at the 17 inches. Back in 1981 the industry was very firmly fixed on 15 and 20 inches, and had been for a long time. You 100% sure on the 17"? It's not 15 and you're measuring it on a diagonal, is it? If so my 15 inch suggestion above makes even more sense
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I'll bow to JFM's knowledge of these things and go with his recommendation.. :)
 
DSCF0345.jpgDSCF0347.jpg
hopefully pics attached which show the measurement, i've measured from where the bracket touches the transom to the cavitation plate, parallel to the leg
 
ortogo said:
I'm looking at getting a new small outboard for my 9foot fibreglass tender to replace a 35 year old 4hp Johnson. The Johnson measures 17 inches from the mounting bracket to the top of the cavitation plate which exactly matches the dinghies transom. The outboards I've been looking at measure either 15 inches or 20 inches, my question is, is it better to go for the longer or shorter length or is it one of those things that won't make much difference in the real world
Click to expand...

17 " from transom top to the anti-ventilation plate will correspond to a standard 15" shaft model.
My 20" Evinrude measures about 22½"

Ideally the AV plate should be level with the bottom of the transom or just a little below but its only critical for a boat and engine combination that will plane

If you opt for a standard, aka short shaft, ( ie 15") be sure that the water pump will be submerged.

If you can cut a section of the transom down to 15" that might be the best option otherwise you may have to suffer the disadvantages of several inches extra depth of a long shaft (20") model
 
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From the pics, that is a short shaft, nominal 15inch. The end of your tape should be at the part of the clamp that kisses the transom's aft face not forward face, so that knocks an inch off ie you would measure 16inches, and then it appears Johnson took a policy decision to deepen the prop a bit so they added an inch. Generally with fast boats you want the cav plate just above the keel line (hence jack plates on v fast boats) and with a v slow boat you want it just below, and Johnson must (I guess) have assumed this 4hp will always be slow so they added an inch

Definitely buy a short shaft 15 inch to replace it. As Vic says you don't want the disadvantages of dragging an extra 5 inches thru the water
 
jfm said:
I'd get the 15 inch. At the D speeds you'll do with 4hp and 9 feet the 2inches too high won't affect performance nor induce cavitation. You may be able to cut 2inches off the transom top edge anyway, if you don't like how it runs.

All that said I'm very surprised at the 17 inches. Back in 1981 the industry was very firmly fixed on 15 and 20 inches, and had been for a long time. You 100% sure on the 17"? It's not 15 and you're measuring it on a diagonal, is it? If so my 15 inch suggestion above makes even more sense
Click to expand...

+1

Standards are 15 and upwards in 5" steps
 
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transon%20height.gif
 
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