Starcraft Islander 221 Mercruiser 140 best prop

[Gunner 1]

Hello , I have a 1987 Stracrat Islander 221 with Mercruiser 140 and alpha 1 drive . My current aluminum prop is marked 15 X 15.5 and its performance is actually pretty good . Wide open throttle is 45-4600rpm and top speed is 30mph on gps. I have smart tabs and my hole shot is okay . My only concern is once I get to 4000 rpm is Im almost doing 29 mph . Giving it more throttle definiitely raises engine rpm but not any real speed gain which puzzles me . Anyone else have this boat engine combo and what prop are you running. Really appreciate any feed back

 

[steelespike]

The Islander 221 is about 1600 lbs dry weight.I think it should be able to handle a 17 or even a 19" prop and turn 4800 rpm. speed about 38.
There are only 2 props smaller than the 15" You would usually find a 15 On a pontoon.Your motor would normally wildly over rev with a 15" prop.
Is your prop a 15.5" pitch or 15" pitch no big deal Diameter usually involves fractions and size is listed diameter X pitch.
Unfortunately I think your motor may be tired.But lets try some things.Check that the throttle opens all the way.Be sure the timing is set right and advancing
as it should.Do a compression check. Is the prop in excellent condition? Is the bottom clean and smooth?
Back off the tabs all the way do a lightly loaded test run get us the numbers.rpm and speed.See if it responds to up trim?
Engage the tabs a little at a time until they are barely enough to help.
They are said to help speed but I'm sure if applied to aggressively could slow the boat.

 

[Gunner 1]

Thanks for the reply and suggestions steelespike. The timing was set , (new points condensor) and I believe advancing . . The boat and prop are in excellent shape and hull squeaky clean . The motor being tired is a possibility as I am third owner. I will recheck my compression this weekend . The small prop diameter confuses me though . I thought I have seen other Islander owners with the 15 inch even some with 14 3/4 props ? My tabs are the self adjusting kind and I have them set in second hole which is the starting point Nauticus recommends. Engine sounds good at speed the main thing I notice is once at about 28 miles an hour 45-4600rpm ..... my last quarter of throttle increase DOES increase rpm but without much speed increase so your under size suggestion may be whats wrong. .

 

[steelespike]

Your prop diameter is a secondary consideration and very basically diameter is somewhat dependent on the pitch.
Very basically on any prop as pitch goes up diameter goes down. Not always and not always the same.At around 16" pitch
diameter is sometimers about 16"
Your 15 1/4" diameter with a 15" pitch is normal.If you look up say a 23" pitch I think you will note the diameter in the 14" area
14,14 1/4, 14 1/2. etc .
Most people have good luck with the smart tabs and usually they are seized right.However think about it your very likely dragging some of the tab through the water and they may not allow the bow to rise to a normal wot form.So you could be dragging too much boat through the water.
That would certainly explain the lack of further speed above 4000.
Does it respond to up trim?
As far as timing just confirm it is advancing as it should it's not unusual for some of these old timers to have problem advance mechanisms.We're grasping at straws to see if we can get your best speed and to try to be sure all is right won't gain much butCould feel impressive with as little as 2 more mph.