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Old 18 November 2008, 04:31   #1
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Wrong prop?

Now having got a few hours under my belt I am wondering whether the correct pitch prop is fitted to my boat. It accelerates nicely up to speed, but going from about 3/4 to full throttle gives no more revs or boat speed. Unfortunately I do not have a tacho so I don't know if I'm already maxxed out at 3/4 throttle position. If the engine has got a few more revs left, do I need to fit a finer pitch prop to take advantage of it?
I will check the throttle movement next time I'm with the boat to see if it is physically wide open at the 3/4 control position...
BTW I have seen 27mph with just me on board - is that ok for a Zodiac Pro 420 with 25hp?
Cheers,
Jason.
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Old 18 November 2008, 09:04   #2
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If you are achieving full WOT at less throttle, correct it, to be OK at full WOT, it's mission impossible to tell max rpm at WOT if a tacho is not placed to test best boat/prop performance if looking for top end speed. Factory delivered engine props are medium sized pitched, assuming your engine was delivered with one, so some sizes available to play in the - to + pitch range. Go for a Tiny Tach, simple to install and efficient.

A 25 HP for that size boat a bit short, much nicer a 30 or 40 HP, 2 strokes. Don't expect great performance with that engine. See if you can convert cheaply your 25 to 30 HP.

Happy Boating
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Old 18 November 2008, 11:43   #3
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You should be able to rig up a tach using car tune-up equipment as a temporary measure in order to see what's happening. A lot of them use an inductive pickup on the spark plug wire, and a connection to 12V. Simple enough, though it means running with the cowl off for a bit.

jky
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Old 18 November 2008, 12:15   #4
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27MPH is about 23 knots. I used to get 21 absolute max with calm water etc etc on my old SR4/ Suz DT25 with 60L fuel & a hefty homemade console, so your Vmax is probably about right for that setup.

Also don't confuse throttle angle with lever angle on the remotes! My recently expired Yam 55 had a funny arrangement with a cam plate in the engine - the actual throttle was at WOT long before the lever hit it's stop - WOT was reached (like yours) with the remotes lever at about 45 - 50 degrees to neutral.
Most throttles on outboards seem to have (or at least had!) a spring arrangement so that if your remotes gave more cable throw than the engine needed it would squash the spring instead of putting strain on the linkages. The DT25 certianly made use of that "feature" with the official Suz remotes.

Speed sounds OK, and the "extra" movement sounds all too familiar. I wouldn't loose too much sleep over it. Incidentally, what prop are you running on it? I had a 13" on the DT.
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Old 18 November 2008, 18:44   #5
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I think 9D280 talks sense here. The speed you are getting is good for the size of boat/engine combo - a different prop *might* gain you 1 or 2 mph extra if you are lucky - probably not worth the effort.

If you are not happy with the speed then you probably need to follow Loco's advice and look at either upgrading or replacing the engine - although I would not agree with his suggestion that you are particularly short of power. 18-20 knots is a sensible cruising speed for a boat your size.
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Old 19 November 2008, 03:42   #6
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Thanks for your useful advice guys. It sounds like I'm getting good performance for my outfit - although extra hp would be nice, I like the economy and quietness of the 4 stroke, and anyway the DF25 is the max weight of the transom. I'm not bothered about going any faster, just want to ensure my outfit is running at its best! (which it sounds like it is!)
I'm afraid I've no idea what prop is currently fitted - is it marked on the prop somewhere?
I will check this spring action out next time I'm on the boat - sounds like what it feels like, and those "Tiny Tach" sound like and ideal xmas pressie!
Cheers all!
Jason.
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Old 19 November 2008, 07:49   #7
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If it's a Suz prop there should be a small boss visible on the hub just behind the prop nut. It will (or at least they used to, assume it;s stil lthe case) have a P followed by a 4 digit number on there. first two are your pitch, second two are I think to distinguish variants (e.g heavy duty).

Failing that it will be written somewhere along the outside of the hub between the blades.
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Old 19 November 2008, 13:31   #8
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Quote:
Originally Posted by Polwart View Post
If you are not happy with the speed then you probably need to follow Loco's advice and look at either upgrading or replacing the engine - although I would not agree with his suggestion that you are particularly short of power. 18-20 knots is a sensible cruising speed for a boat your size.
By short was reffering that mostly all 420 sibs uses 30 HP as max engines, Seems Suzuki does not produce a 4 strokes 30 HP engine, next is a 40. So if you don't plan to transport many mates/gear/weight a 25 will work nicely on a lighter sib arrangement.

Happy Boating
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Old 19 November 2008, 14:34   #9
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Mostly just me or one other. I go boating for the journey! more time on water=more fun, so 25 horses is fine for me!!! (The wife has lots of fun with one horse, though it does have more legs than my RIB...)
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