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Old 13 March 2006, 14:12   #1
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Prop choice

My new boat 8.8m 225hp 2 stroke in case anyone forgets.....

Currently there is a 4 blade 19" pitch stainless prop fitted - should be great for acceleration and carrying 12 people but prob won't give much of a top speed.

Have just bough a lovely suzuki 24" stainless prop off ebay - 3 blades. May be a bit too fine a pitch though - to complicate matters there is a transom jack.

Just wondering if the 24" prop is going to be suitable - even empty the boat will have 500l of petrol on board. I suppose it's just a case of try it and see???
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Old 13 March 2006, 14:19   #2
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Codders you know prop selection is a bit of a black art so 'suck it and see' is a good approach. However, gut feel from me would be that 24" would be too coarse a pitch and that you wouldnt be able to pull the right RPM's and would be straining the engine. IIRC Blue Ice (7.5m/200hp) runs either a 21 or 23" prop - I think Richard went to the 21" after experimentation. I am not sure how much difference the jackplate will make (ask Jonny F or Cookee as thats more in their area of h'expertise), but as I say I reckon 24" is too high. Your 19" will get you from CI to the mainland - a trip where you will be fairly loaded down with cheap fuel yes?
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Old 13 March 2006, 14:21   #3
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Codders, I wouldn't worry to much about it until you get to try it, I assume you will be running it in on the way home so there won't be much need for flat out.
Once its run in you need to see what revs its pulling at WOT and check that its within Suzuki's recommended range (5400-5600 I think on my engine), yours should be in you Suzuki manual you bought of ebay!
Best thing to do is test it light and loaded, but because your boat is so unique with the transom jack etc, nobody can probably tell you.
You may wish to have two props one for when your carrying people and one for when your light, to test loaded you could fill up a few drums of waters and see how she performs or just find 12 people, count me and Hugh in
You have two props to play with and a decent propeller company (steel developments, propeller revolutions) could repitch either of those.

Alex
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Old 13 March 2006, 14:40   #4
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Quote:
Originally Posted by codprawn
Currently there is a 4 blade 19" pitch stainless prop fitted - should be great for acceleration and carrying 12 people but prob won't give much of a top speed.
I work out your max theoretical speed with that prop to be 47 knots - actual speed will probably be in the low 40's

but it sounds to me like you are carrying a lot of weight and you may find going up any higher pitch you will struggle to reach max rpm.

what does your boat weigh without the fuel - and will you always be carrying 500kgs of fuel around - i would try to only fill her up when you have a long trip to do its pointless burning fuel just to carry fuel...
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Old 13 March 2006, 16:01   #5
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I thought 24" was a finer pitch than 19" or - have I got it the wrong way around?
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Old 13 March 2006, 16:10   #6
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IMO coarse and fine are the wrong words to describe the pitch of a prop - long and short are far more descriptive or steep and shallow

you would describe a thread on a bolt as course or fine - so if you do use those words then a 24" prop would definitely be coarser than a 19" as a coarser thread pulls a bolt further per revolution so a coarser prop would pull a boat further per revolution
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Old 13 March 2006, 16:25   #7
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No wonder I get confused - as you say much easier with a bolt - 4 tpi would be really coarse and 240tpi would be really fine.
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Old 13 March 2006, 16:34   #8
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Quote:
Originally Posted by codprawn
I thought 24" was a finer pitch than 19" or - have I got it the wrong way around?
24" is a larger pitch than a 19"

A 24" prop would move 24" inches through the water in one revolution wheras a 19" would only move forward 19" in one revolution.
At least thats my understanding.

With a lifter i'd guess you could maybe go up an inch of pitch over a similar boat without the lifter.

Harry
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Old 13 March 2006, 16:44   #9
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41knots to be exact.

Dom
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Old 13 March 2006, 17:54   #10
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Quote:
Originally Posted by codprawn
Just wondering if the 24" prop is going to be suitable - even empty the boat will have 500l of petrol on board. I suppose it's just a case of try it and see???
Congrats on the new arrival, before I forget.

19" to 24" is quite a change. General rule of thumb is 200 to 250 rpm for each inch of pitch change (down in pitch = up in rpm.)

What I found in my limited testing is that the rule of thumb is an extremely rough estimation. Mine went from 19" pitch (turning about 4000 rpm) to a 13" (which should have put it just over 6000rpm) and couldn't get a reading as it sat on the rev limiter at about 6100.

Ended up with a Ally 15", turning a tad over 6000 with just me and a less-than-full fuel tank.

Now running a SS 15" which turns about 5800 with a light load, about 5500 to 5600 with a "normal" load.

So, whether or not the prop will work for you depends on a lot of things, not the least of which is where the 19 falls in the rpm range. The 24 should put you somewhere around 1000 to 1500 rpm higher, in addition to less of a hole shot, and a substantial increase in top speed (assuming you don't lug the motor down.)

jky
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