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RPM VS SPEED on 24ft Starcraft Islander


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Hi All

 

I was wondering if I could get the Islander owners out there to let me know what speed they get at 3500 RPM

 

I have a 1989 Starcraft Islander 24ft with a 4.3L Mercruiser (approx 190hp) with a alpha 1 drive,

 

I Get 27 mph at 3500 RPM and 31MPh at 4100RPM

 

The reason I'm asking is I checked the compression and I'm over 150psi in each piston after I rebuilt the engine in the spring and I'm wondering if I'm getting all the performance I should be from it?

 

This is my first I/O and I have no clue what I should be getting out of it

 

Any input would be appreciated

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The Maresk company that ships containers all over the world has completed its newest line of ships. As large as our aircraft carriers which speed over thirty knots, the new ships are engined with smaller engines that cruise at sixteen knots, top speed eighteen knots. Fuel is a huge expense for them and the best economic model they computed is that speed. How much speed do you really need? 

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It's not really about speed but fuel efficiency. If I have to use more RPM to get my boat up on plane then it needs I want to make adjustments. I 've heard of some people getting 30mph at 3000RPM  with the same boat. Since I know that my boat will plane at about 20MPH then they can achieve something at 2000 RPM that I can only do at 2600 RPM. That equates to hell of a difference in Fuel at the end of the year. I don't believe the Maresk company is concerned about getting something the size of a aircraft carrier up on the last 3 feet of stern for fuel economy

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Just my 2 pennies.

You can only make a comparison if all the other factors are the same.

Like overall weight,weight distribution,trim tabs, cleanliness of hull,size of propeller,and so on.

So no matter what the other guys do,it may or may not be possible for you.

My Islander is a 19 footer with trim tabs and a 3 liter engine.She goes up on plane at 16mph if I trim her the right way.

If I don't trim her the right way I do not make it to 16 mph and all my fuel goes toward creating a very large bow wave.

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My 22' planes out at 17 mph run at about 25 at 3400 rpm. Trim the outdrive and the speed picks up I have a dolefin on it and it helps the ride. I am more interested in my trolling speed rather than getting there quicker. The right trim will help your fuel miles per gallon. And a good tune up.

Prop pitch and things like cupped blades on the prop will change the rpm at a given speed.

Edited by Big Dave
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I have a 22ft Sea Ray cuddy with stainless steel prop, with 350cid I can't tell you what my get on plane speed is because when I open the throttle she jumps on plane and I'm going 20mph already. I troll with my main engine, so I'm not worried about the extra mouth full of gas it took to get on plane, at 2800 rpm everything seems to come together motors not working hard and I'm cruising 25mph and at 5000 I" cooking along at 48mph but man she drinks fuel like it's margarita time. I only do that when I'm being chased by mother nature, sometimes I win sometimes she wins, if I had a 80mph bass boat I would win more often LOL

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I would think you would want to prop the boat so at WOT your RPMs would be the manufacturers recommended   4400-4800. The best way to stay on plane at lower speeds would be Trim Tab's or adding a fin to the outdrive . I added a SE 300 sport hydrofoil to mine , helped with hole shot and staying on plane at lower rpm's . 

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I would think you would want to prop the boat so at WOT your RPMs would be the manufacturers recommended   4400-4800. The best way to stay on plane at lower speeds would be Trim Tab's or adding a fin to the outdrive . I added a SE 300 sport hydrofoil to mine , helped with hole shot and staying on plane at lower rpm's . 

like what jack said on this post, 4800 max rpm.  custom props can be fabricated to your boating needs.  thers lots of differences that can make a perfect custom prop.  only problem is even with the science and math perfect, thers always a variable and your first custom prop may not work the way you thought it was designed and the second and sometimes the third is the charm but it can be expensive.  if that doesnt sound like you then your better off loadin your boat up the way it was intended and pitching to 4800 rpm W.O.T but stick to a large 3 blade for gas mileage.  4 blades add some resistance to the water at cruise speeds but give traction making you think there faster.  same pitch 4 blade should be same speed as 3 blade.  if the 4 blade is faster mph at the same rpm then you had too much slippage in the water.  propping is a crap shoot unless you have some ideahs on where your at with slippage and hull friction on your exact boat.  sooooooo many variables.   take trial and error or take the 4800 wot recomendation and leave it alone.

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