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Old 30 May 2019, 09:17   #1
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Prop pitch

Forgive me, this has been discussed a lot, but I can't for the life of me find my answer using the search function.

We were out on the Honwave (T-38) last weekend seeing the seals off Morston in Norfolk... There were 3 adults and 3 young kids... and a dog... so we were technically over capacity but perfectly safe for the conditions... getting any speed up was impossible and understandably so.

So... I'm off to Devon in the summer, there will be 2 adults and 3 kids (and one dog) going out a fair bit and I'd like to get on the plane.

I have a Honda BF20 with standard prop. I assume I "might" get on the plane but I also suspect I might not due to the stature of some of my fellow passengers.

People talk about propping up or down, changing the pitch to drop down a gear and thus help get a heavier load up onto the plane.

What prop would I need to buy to achieve this?

Thanks...

Stig
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Old 30 May 2019, 09:41   #2
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Check out the markings on the existing prop... probably a 10" pitch. You might drop to a 9" which effectively is like a lower gear in a car.
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Old 30 May 2019, 09:53   #3
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Prop states 9 1/4 x 10 so I assume you're correct there Fenlander

So this should help:

https://www.ebay.co.uk/itm/Aluminium...frcectupt=true
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Old 30 May 2019, 10:30   #4
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this will help but you need to know you RPM at WOT but dropping to a 9 dia will give you 100 rpm higher ish and dropping to 9 pitch will give you 200 rpm higher ish might be enough to get over the bow wave

on this site they do a high thrust prop which might suit you for the full boat





https://www.propeller-shop.eu/propeller-info/
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Old 30 May 2019, 10:33   #5
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Quote:
Originally Posted by jeffstevens763@g View Post
Very helpful, thank you
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Old 30 May 2019, 10:42   #6
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Yes it’s got to help. Unfortunately advice and calculations can only be a guide... only getting on the water with the new prop and load will tell you.

I’ve never needed to go below s 9” for a 15 or 20hp on a sib..
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Old 30 May 2019, 10:48   #7
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Yes itís got to help. Unfortunately advice and calculations can only be a guide... only getting on the water with the new prop and load will tell you.
yep got to agree with david the 1 inch pitch lower is estimated at 150 - 200 more rpm IME it works out at 150 ish just moving folk about makes hell of a difference, bit of wind, tide and the calcs go through the window hi thrust props help tremendously for load carrying worth looking into but RPM needs to be known really
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Old 30 May 2019, 11:19   #8
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but RPM needs to be known really
Well when we were out at the weekend it felt like the prob was spinning in treacle... I doubt the RPM went above 4k and she's normally WOT at close to 6k.

P
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Old 30 May 2019, 11:29   #9
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Originally Posted by Stigomery View Post
Well when we were out at the weekend it felt like the prob was spinning in treacle... I doubt the RPM went above 4k and she's normally WOT at close to 6k.

P
yep she needs a bit to get over the bow wave and that doesn't calculate because its unknown you might drop an inch of pitch and be ok with that load or it might need 2 inches.
if it were me i would invest in a tacho take it on hols and load the boat up in bits to find where it starts to slump and work from there in the meantime ask about high thrust worth a phone call, steel developments are good to talk to.
looking at your load and again how much each person weighs you might be an adult too much that could be 2 inch of pitch reduction as a gess happy to be wrong though
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Old 30 May 2019, 14:23   #10
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T38 Best Water Performance.-

Riding a T380 well crowded with 6 up + a dog with a 20 HP motor is not the way to go at least for me. If wanting to get the max out of that combo will need to inflate it to factory recommended working pressure, have the motor sitting at the sweet motor/transom height, well trimed and with deck weight evenly distributed to start with.

Motors delivered with a factory installed prop are usually medium pitched, factory wants to play safe on their side as a precaution and doesn't know which size boat this particular HP motor will be powering it.

In order to prop any motor right for top water performance need to install an induction tach, go for a wot run on calm, flat water cond as usually loaded and check what was the max rpm achieved, compare those numbers to the min-max wot rpm range factory stated which is ? Motor needs to rev inside the safe rpm range, if revving towards the min range, the motor is being highly stressed, ideal is to run middle to max wot rpm. Pros and cons of both :

Middle range achieves a slight faster top end speed but sacrifying best hole shot, max range achieves a faster hole shot with load and slight less top end speed. Your choice which one you like best. For me full wot revs as lightly loaded, 2 up, no dog.

One issue to bear in mind is : will original poster (OP) be boating constantly with 6 up + a dog ? If you prop right the motor to run say middle to max wot rpm range for that particular situation, when with less passengers onboard the motor will overre, so will need to throttle much less if liking running full throtle.

Without a tach reading won't know how many revs will motor need to work inside the wot safe range, for that adventure need a less pitch prop from current installed one, the tech question is by how much less ?

The stated 100-150-200 wot revs per pitch up or down is not cast on stone, only dials with heavy loads, with medium light to medium loads can achieve as much as 300 to 500 wot rpm increase.

If plan boating as stated won't know if in need to drop one/two full pitches, a tach will tell, go for one..

Happy Boating
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Old 30 May 2019, 14:59   #11
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Locozodiac are you saying 300-500 rpm per inch of pitch light load if so I don't agree in my expeariance 150-200 rpm increasing weight just means you need to gear down to keep rpm up loosing WOT top speed to a certain extent I do agree it's not an exact science and a certain amount of trial and error. The op has the dilemma a of a large family crew at displacement speed say 7-8 knots he will probably plane at 10-12 knots I would pitch for that because under normal weight he will run at about 18 knots with the current prop so coming down 2 inch on pitch might get him on the plane but I wouldn't bank on it due to not knowing the exact details. All IMO
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Old 30 May 2019, 17:39   #12
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When fine tuning an outfit for max speed at appropriate revs I agree a tach is almost essential. But just to give an opinion to get a sib on the plane that otherwise won't climb the hump is a bit more basic I reckon.

Honda only supply from 7" to 11" pitch for the 20hp and Stig is saying the 10" is too high so an 11" will be worse. That only leaves 7, 8 and 9. I'd say a 7" would be great for a displacement boat and for Stig's situation that leaves an 8 or 9.

In real life 1" reduction makes more difference than it might seem on paper such as when my 20hp came with an 11" which felt sluggish and I immediately bought a 10" which improved it massively.
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Old 31 May 2019, 03:53   #13
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Thanks again chaps...

So I do have a tach, one of the first things I bought... reads 1/2 values though

With a light load, in perfect conditions, WOT will deliver me a smidge over 21 knots at around 6k rpm

My normal max load is 2 adults and 2 kids with the usual kit, achieving planing speed is easy with this load... more often than not it's just me, the boy and the dog.

So this new prop will be for the occasional friends visiting and wanting to all go out together... but not at displacement speed if at all possible...
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Old 31 May 2019, 04:21   #14
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looking at the sums for a new prop dropping an inch £55 if it doesn't work you still have a spare prop for the majority of your boating it then lends itself to do you want to spend another £55 when you take your mates out for another inch drop.
just remember if everything is based on perfect conditions it's a different ball game in the rough
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Old 31 May 2019, 04:24   #15
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Quote:
Originally Posted by jeffstevens763@g View Post
do you want to spend another £55 when you take your mates out for another inch drop
Well, if last weekend is anything to go by, and that inch makes the difference in getting us over the hump, then yip... it's worth it...

I will report back mid-summer!
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Old 31 May 2019, 04:28   #16
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Well, if last weekend is anything to go by, and that inch makes the difference in getting us over the hump, then yip... it's worth it...

I will report back mid-summer!
go for it
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Old 31 May 2019, 15:54   #17
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As an example of what have posted: Have a 2 strokes Tohatsu 18 HP motor that's been powering several sibs and ribs as well, just playing with prop pitches when going for the larger ones. Now own a Rib.

450 Rib characteristics:
Dry weight with motor, fuel, cold ones : 300 kilos
With 2 up medium loaded: 500 kilos (half ton)
Factory wot rpm range : 5200-5800
Factory delivered OEM prop : 9 pitch
Max wot rpm achieved : 5500 RPM
With an OEM 8.5 pitch prop :
Max wot rpm achieved : 5800 RPM
Motor is running at its safe max wot rpm range factory stated.

If lightly loaded one up (me alone)
With same 8.5 prop : 6100 RPM
That's + 300 wot revs with just half pitch decrease.

Have tested 2 high quality induction tachs and both read same, so don't know where do you get just 150-200 revs difference unless your tach is way off, inaccurate, setting is incorrect, run a boat that has extreme hull drag, was tested over crowded or went from a OEM prop to a different prop brand.

Each boat is unique and boating with varying loads on board is a RPM serial killer compared to a fix load which I ran and motor propped right to run towards max wot rpm range. When boating alone have to throttle bit less or motor will over rev, that's the neat issue about counting with a tach installed on tiller arm close to the throttle grip.

Happy Boating
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Old 31 May 2019, 16:45   #18
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LZ interesting results but you have different hull caratistics from the OP he has a SIB you have a RIB but great Input

Cheers
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Old 31 May 2019, 18:19   #19
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Another Prime Example.-

Quote:
Originally Posted by jeffstevens763@g View Post
LZ interesting results but you have different hull caratistics from the OP he has a SIB you have a RIB but great Input

Cheers
In order for any motor to achieve the best prop thrust prior going for a prop maximization while powering any sib you must meet these spot on working conditions :

-Sib must be fully inflated to its factory working parameters with a pressure gauge.
-Motor must sit at the sweet transom height.
-Motor trimmed to 90 deg. on transom.
-Deck weight must be evenly distributed, same as lower picture...

Tested a 380 alum floor Sib with a 2 stroke Tohatsu 5 HP motor.
Factory WOT rpm range : 4500-5500 RPM.
Total combo weight with 1 up max 200 kilos.

With an OEM 3 blade 8 pitch prop (factory delivered) was impossible to plane, tested a OEM 3 blade 7 pitch, max wot rpm achieved was incredible 5500 rpm, now 380 combo planes easily with 1 up and at fast boring displacement speed with 2 up.

Motor gained +1,000 revs with one less pitch. Besides the mentioned Sib example now pushing happily from time to time a J24 sailboat which weights 1.5 tons at 4500 revs while at middle throttle at 6 knots/hour on flat calm water cond.

Not saying that all prop brands will gain such oustanding rpm numbers, will depend on how each brand manufactures their own props or have them manufactured by a third party with their tech specs. Only a wot run with a tach will tell the rpm difference between each prop brand going say +1-1 in pitch.

Happy Boating
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Old 01 June 2019, 03:31   #20
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Stig - if you are looking for a new compromise prop that still works well lightly loaded and might get you over the hump with a very heavy load then dropping an inch is probably a good plan. However if you are saying you want a prop for very heavy loads but will swap it for your existing one when normal operation, then Iíd drop two inches. Youíll probably max out about 16 knots but have a far higher chance of driving the boat up hill to get to that speed.
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