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Old 10 December 2019, 03:34   #1
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Mercury 90hp Saltwater - 4 blade prop?

Hi,

I have an Avon SR5.4 with a Mercury Saltwater 90hp 2-stroke swinging a Solas 13x21 aluminium prop. Top speed with this rig is about 37kts.

I'd like to change to a 4-blade prop to improve hole-shot (flooded hull) and grip; and reduce torque on the steering without sacrificing too much top speed.

Can anyone recommend an appropriate 4-blade prop for my rig? Would prefer stainless steel.

Thanks in advance!
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Old 10 December 2019, 04:33   #2
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you will get better hole shot with a 4 blade prop but lose a bit on top speed,steel developments are worth talking to


Basic Propeller Info
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Old 10 December 2019, 07:10   #3
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The mercury spitfire prop is supposed to be very good and apparently doesn't have the usual top end loss of a 4 blade. they come in Aluminium and steel, although steel seem harder to get in the uk. Don't forget you may need to change pitch when you move to 4 blades.

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Old 10 December 2019, 08:19   #4
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But what pitch I am supposed to go for?
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Old 10 December 2019, 09:57   #5
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But what pitch I am supposed to go for?


Whatís your WOT rpm with your current prop. What is the optimum WOT rpm for your engine?
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Old 11 December 2019, 02:22   #6
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WOT range for the engine is 5000 to 5500 rpm, at full chat the she does approx. 5200 rpm.
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Old 11 December 2019, 04:47   #7
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personally i think you bang on with that prop, do you require the flooded hull i know some block them off.
if you go stainless same pitch they are reputed to be 6% more efficient and might give you what you need.
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Old 11 December 2019, 05:10   #8
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If it's just hole shot you're after I'd stick with the same pitch & diameter but go for a vented prop. eg Lazer II. That will let the engine rev a bit more at pre-planing speed.

Try and borrow one to try it first.
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Old 11 December 2019, 05:23   #9
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If it's just hole shot you're after I'd stick with the same pitch & diameter but go for a vented prop. eg Lazer II. That will let the engine rev a bit more at pre-planing speed.

Try and borrow one to try it first.
interesting every day a school day explains below for info

https://www.boats.com/pvs-propeller-test/

trying to get my head around the flooded hull only seen one on a 4 m searider but i'm under the impression that the water has gone in a boat length so long as air can replace the water quickly or does it take much longer for some reason?
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Old 11 December 2019, 18:50   #10
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4-blade would also improve grip and decrease torque on steering I understand.

Still not sure what size/pitch to go for in a 4-blade. Any suggestions?
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Old 11 December 2019, 23:35   #11
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Have found:

Solas HR Titan 4, either 13x21 or 13x19
Solas Rubex HR 4, either 13x21 or 13x19
Mercury Spitfire X7, 12.7x21, has Performance Vent System (PSV)

So perhaps the Spitfire X7 12.7x21 is the best option as:

-slightly lower diameter compensates drop in WOT due to 4th blade and cupped design?
-PSV may increase RPM at lower speeds for better handling?

Followed by one of the Solas 13x19 as:

-rule of thumb is to go down 1" of pitch at same diameter when going from 3- to 4-blade
-lowering pitch by 2" (21" to 19") in this case could bring up WOT too much
-cupped design could bring WOT down again (equivalent of 1" more pitch and bingo?)

What do you think?

Also, what is your experience with the Solas Rubex hub? Any pros or cons?

Lastly, for the price of one Spitfire X7 I could get two Solas props... :-/
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Old 12 December 2019, 06:00   #12
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Laser II

I would think that the optimum prop. for your set up is a 3 blade laser II 13 1/4 x 20p, Mercury part no. 48-16990 20P, you will get about 5400 RPM @ WOT x circa 40-42kn. with reduced torque feedback via the wheel.
I'm assuming that your engine is a 2 stroke with possibly an XL (25") shaft.
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Old 12 December 2019, 08:46   #13
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Hi,

Have you tried the Lazer on a similar rig?

Best,
A.
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Old 12 December 2019, 10:00   #14
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Yes,I had it on the same boat with the same 2 stroke 90hp Mercury Saltwater Series. It had the same flooding hull, virtually instant planning, ie quick 'holeshot'.
With 50L fuel & not much else in the same boat & with the engine trimmed out to the limit in flat calm water, the boat would happily do 41 / 42kn. but would chine walk very easily if disturbed.
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Old 12 December 2019, 10:31   #15
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How about grip at cruising speed with higher load and torque on steering? The latter I find really tiring.
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Old 12 December 2019, 10:44   #16
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Quote:
Originally Posted by Appleby View Post
How about grip at cruising speed with higher load and torque on steering? The latter I find really tiring.
You should be able to set the trim tab on the gearbox to eliminate the steering torque at your cruising speed. Pull one way above and the other below.
I'd go with the laser 3 blade option from a "cost risk" point of view. You'll get a second hand one for sensible money.
The blade rake will mean you might have to take an angle grinder to the trim tab as the blades might contact it but as I said you should be able to get rid of the steering toque by adjusting the steering tab.
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Old 12 December 2019, 10:48   #17
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The Laser prop. gripped well, (I think it's slightly cupped), I don't remember it ever loosing grip except when trimming excessively out to gain max. speed (though that is more cavitation). There was less torque feedback via the steering versus the aluminium prop, but still some as I recall. I have the prop, at home on my window sill for years as a paperweight!
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Old 13 December 2019, 04:48   #18
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My 5.4 had a 2 stroke Mariner 90hp and a 20" Laser 2.
Did close on 40 knots (Subsequently found that the engine was an inch too low so expect would have done a little more if raised).
No issue with fitting - ie no conflict with the tab on the engine.
My hull was still the flooding as built and no issues with getting the water out.
From a standing start if you floored it you got a very high bow. The way to drive the flooding hull boats is to get them to a speed where the water is flowing cleanly from the transom - about 7 knoys and then the water drains out. When you get to where you want to get up to speed you're already ready to go.
I plugged the vents in my propeller.

Note that there are two hub sizes of laser 2 propeller. You'll need the small hub rather than the large hub for V6 outboards and Alpha drives.
Small hub propellers come in even pitch sizes, large hub come in odd pitch sizes.
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Old 13 December 2019, 10:33   #19
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Quote:
Originally Posted by Appleby View Post
Hi,

I have an Avon SR5.4 with a Mercury Saltwater 90hp 2-stroke swinging a Solas 13x21 aluminium prop. Top speed with this rig is about 37kts.

I'd like to change to a 4-blade prop to improve hole-shot (flooded hull) and grip; and reduce torque on the steering without sacrificing too much top speed.

Can anyone recommend an appropriate 4-blade prop for my rig? Would prefer stainless steel.

Thanks in advance!
Regarding the torque on the steering there shouldn't really be any. Mr SR5.4 with 90hp Mariner had cheap cable steering. You need to play with the trim to get rid of the torque. As you trim up from full down the torque will reduce and then increase but in the other direction. The boat generally runs best when the trim is adjusted to give no weight on the steering.
My engine was mounted about 1-1.5 inches to starboard of the centreline
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Old 17 December 2019, 15:46   #20
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33JS has a 20" Laser 2 for sale in the classifieds.
They don't come up for sale very often.
I'd recommend you buy it and try it. If it doesn't do what you need you can easily sell it on.
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