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The Norfolk Broads Forum / Broads Boat Owners Q & A / New engine gearing
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Posted By Discussion Topic: New engine gearing

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jimfin
Oct-02-2017 @ 9:30 AM                           Permalink
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We have fitted a new 46hp engine to our 35 foot aquafibre with a sliding roof.
It has a 3 blade 16 - 13 prop fitted at the moment and I was thinking would a 17 inch prop be better.
The engine has max revs of 3000 and I can run it up to 2800 revs under power with no problem.
I normally cruise at 2000 revs.
Would it be worth the bother to change to a 17 inch and would the engine be able for it.

If you don't use your boat. Sell it.

Stingers
Oct-02-2017 @ 10:15 AM                           Permalink
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Jimfin, I'm no expert on this but, the fact that your max revs under power are 2800 on an engine rated at 3000 revs suggests to me that you already have a suitable prop fitted. If you go up to a 17" prop then your max revs will decrease due to the greater load and you will also find that your minimum speed (tick-over in gear) will increase.

If you do an online search for 'boat propeller calculator' you will find several available. These are rather general in nature as they cannot make allowances for all types of hull and the only way to be certain is to try changing the size and see what result you get.

Andy

jimfin
Oct-02-2017 @ 10:28 AM                           Permalink
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Thanks Andy. This is what I was thinking myself. I was just woundering if any of the guys on the forum has a boat like mine. And what prop they are running to what engine power!!

If you don't use your boat. Sell it.

annville
Oct-02-2017 @ 11:56 AM                           Permalink
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Hi Jim The aim is to get the max revs less a tad,ie engine max 3000rpm if on flat deep water 2900rpm would be ideal,A larger dia prop is more desirable rather than a coarser pitch, a pitch of 17" max is often a good starting point,if the pitch is to coarse the slow speed and reverse control becomes difficult.You need at least a minimum of 2" clearance between tip of blade and bottom of hull, otherwise you will get cavitation which causes wear and noise.Tug boats have a larger dia prop for better pulling power but of cause they dont need to go fast.If your engines max rev is 3000rpm i would expect the normal maximum fuel economy less stress,quietest revs to be around 2100 rpm.If you have to small a dia prop it needs higher revs = more disturbance ie outboard engines.There has been two apps on here just lately that seem to work ok.if you intend to buy a prop a AXIOM prop from Northampton work very well they a built heavier and work better in reverse they have a more square blade then normal,he will also work out the best size for you.John .

pargeandmarge
Oct-02-2017 @ 5:35 PM                           Permalink
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Hello All
Slightly off topic a good friend of ours had a light plane with a wooden prop. He said it seemed to rev to hard at cruising speed.
To remedy the problem he fitted the same size prop with a greater pitch.
You could almost smell the the difference when he tried to take off on the grass strip that was always used. It nearly never went skyward.
Cruising was better with a reduction in engine revs.
Landing again a trauma as the engine was nearly stopped  and freewheeling to cut the speed.
That prop now lives over his fire place and the original is back on the front of the plane where it belongs.
Sorry to take us elsewhere but may be the same rules apply.
Regards
Marge and Parge

jimfin
Oct-02-2017 @ 5:57 PM                           Permalink
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I had a look at that "calculate your prop site and it wants to know what weight my boat is!! Anybody got any idea??

If you don't use your boat. Sell it.

annville
Oct-02-2017 @ 6:39 PM                           Permalink
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Hi Jim Try one of the apps the hull length x width x draft = displacement,  1 cubic ft = 10 lbs it will work out the weight for you, it's only approx but gives you rough totell weight of your boat and contents.John


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