Speed increase options

Blylock

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I have a 2002 Wellcraft 21' , 5.0merc carburated, 220hp.
i believe my prop is a 14.5 with a 20 deg pitch, aluminum. i cruise alot with my buddy, he tops out around 60mph, im about 48mph@4800 rpms, which is according to my manual, is max for that motor. i usually pull only tubes, or cruise around. what are my options to increase my speed? i also have an alpha one drive. any help would be appreciated.
 

Its_Only_Money

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If the engine is pulling its max rated rpm at WOT (wide open throttle) then it sounds like you are correctly propped and there is no more speed to be had. You could try a coarser prop (which would drop the rpm at the same boat speed) but this should just prove the original is correct. If with a coarser prop you still get 4800rpm WOT then you can go coaser again until WOT rpm starts to drop. Any increase in pitch will mean you have slower acceleration and pulling tubes etc won't be as easy for the engine (depending on how good it was before and how loaded is the boat), but basically you will trade acceleration/pulling power for speed if you change for a coaser prop. 1" (increase) of prop pitch change should make about 400rpm (reduction) difference to the "gearing".

Sounds like you buddy has a smaller/lighter boat and/or a bigger engine?

Oh and your prop is probably 20" pitch rather than 20deg.
 

capnjack

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Everything It's Only Money said...../forums/images/graemlins/cool.gif

You sound like one of our friends from across the pond? Welcome. /forums/images/graemlins/smile.gif

There is a little more you can do but it may go beyond the scope of trying to provide the solution via this forum.

First of all, check the design speed of the hull with Wellcraft themselves, either by e-mail or by phone. It sounds as if you have all the speed for a 220hp in that hull. However, Aluminium props can flex, so a Stainless with the same pitch would probably bring your revs down a tad at WOT but increase your max speed by a knot or two. I completely agree with I-O-M, his advice is spot on, if you want to increase your speed dramatically your hole shot and mid range will suffer.

When we used to race, we started by looking for top end, so surface props became the norm, but for 150 - 225's etc. that have to navigate buoys we soon found that WOT was not used as much as mid range. So newer four and five blades with less surface area to cut down on drag were employed as they still are today.

Just to recap then, at 4,800 revs you are at max for that engine, however, 4,500 is still ok and a 20" pitch aluminium will probably loose grip at speed which is why you are at 4,800rpms. As I said the next stage is really beyond the scope of questions and answers on this forum. If you are in the US, then Michigan Wheel or Propeller Solutions would be a great place to start asking further questions, or in the UK try http://www.teignbridge.co.uk/ (part of the mighty Brunswick now) or http://www.castlemarine.co.uk/ .

Hope this helps, good luck
Capn Jack
 

spannerman

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If you really want to go faster then you can swap the manifold for a balanced dual plane one instead of the original Merc one and fit a more modern carb and play around with jet sizes, also dump the flame arrestor and buy an easier breathing (marine rated) one.The americans have lots of these type of accessories to choose from being the land of petrol heads, more drastic is you can change the camshaft for one with a different profile depending on where you want the grunt to come in. Did all this with my 5.0 LX and it ran even sweeter than before and the economy didn't suffer too much.
 
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