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Old 04 December 2006, 02:22   #1
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Country: UK - England
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Engine: Yamaha 90
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prop size

Hi, i just got my new Valiant 520 rib at last and took it out for the first time yesterday (sunday) in poole harbour and probably couldn't have chosen a choppier day for my fist outing. I loved it though. Could you tell me what the proper size propeller should be for my engine - A Yamaha 90 two stroke? Also can anyone tell me where the vacuum hose goes to on the engine from the speedo? Thanks Tom
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Old 04 December 2006, 09:34   #2
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'Also can anyone tell me where the vacuum hose goes to on the engine from the speedo?' Thanks Tom.
should be a hookup on the front of the LU.
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Old 06 December 2006, 16:53   #3
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I run a 510 Gemini with 90hp Mercury & it is best with 22P prop. Gained 8mph top speed over 17P & still has good take off as the boat is only 300kilo.
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Old 06 December 2006, 17:37   #4
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Hi, Thanks for your info on the prop. I know nothing about propellers. What does the 22p mean? What diameter is the prop etc? These may be stupid questions but i gotta start somewhere! I'd appreciate your comments.
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Old 06 December 2006, 20:17   #5
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For the speedo question, there should be a very tiny hole in the leading edge of the lower unit. That will be connected to a small plastic or rubber tube that will exit at the top of the lower unit. Your speedo tube will connect there (should you wish - more on this in a bit.)

As the motor moves through the water, you get an increase in pressure within the tube, and that's what the speedo reads. More pressure, more speed.

Problem comes from one of 2 things: Either the tiny hole gets clogged (and it doesn't take much to stop it up), yielding either a false reading (worst I've seen was 18mph while sitting on the trailer) or no reading at all, or the hose pops off the speedo, creating a little tiny firefighting unit within your console.

As such, if you've got GPS, you're much better off, from a speed over ground standpoint. If you absolutely have to have speed through the water, then by all means connect it. I personally prefer a paddlewheel speed sensor (though in truth, I've got all three.)

Prop selection is a little different. You should select your pitch for WOT rpms at your normal load, then choose within that range to find one with acceptable performance (actually, in the US, we usually select based on WOT rpm only, unless that particular props' performance is really wonky.)

Quick definitions: Pitch is the distance one revolution of the prop will drive it forward, in a solidish medium (no slip.) Usually measure in inches. Diameter is just that. Other variables will be material (usually stainless steel or aluminum), number of blades (usually 3 or 4), amount of cup (usually only on SS props, and wildly variable, with, as far as I know, no quantitative measurement available.)

So, load the boat up as you intend to cruise, open it up, and trim the motor up until it starts ventilating (sucking air into itself.) Drop it back down a tad, and see where the rpms are. Ideally, you want the upper part of the manufacturers suggested range.

Very generally, you can use the following to select prop pitch (the rules are a guide, but don't expect exact results, especially if you're making big changes):

***************
General rules for props:

switch from aluminum to SS (all dimensions are the same) should drop WOT RPM by ~100
an increase in diameter of 1/4 inch should drop WOT RPM by ~100
an increase in pitch of 1 inch should drop WOT RPM by ~150 (range is 100 to 200)

3-blade vs 4-blade
4 blade props have some advanteges over 3 blade. If you have like material, then 4 blades promote stern lift, give better mid range fuel econ, slower top speed, better holeshot, are more balanced and dont vibrate as much and allow you to mount your engine higher due to more blade surface area in the water.

Generally, the heavier the boat the more blade surface area you want, not more pitch. So, rather than going up in pitch, go up in diameter or number of blades or a combination of the 2. This promotes less slippage, but costs you top end. Make the boat more efficient in cruise rpm range.

These are REAL general. You must consider that manufacturer A's prop might have identical dimensions as manufacturer B's prop, but there will be a large difference in WOT RPM's. This is usually due to the variation of blade width, cup, hub flare... There are many things that can cause a difference.

The other thing that could cause such a difference is the current at the time of your test. Maybe if you were able to run the other direction with the props in question, you would see a speed decrease of 5 mph. This would make your average lower and affect your calculations. With out the second pass, you shouldn't count the data. unless you can compare the one way pass with these props to the same direction one way passes with all the other props. Then the comparisons would be level.

Edited by physicistkev 4/26/2005 9:33 AM
*****************

The above was picked up off the net somewhere, so thanks, Kev, whoever you are.


Hope this helps;

jky
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Old 07 December 2006, 04:55   #6
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Quote:
Originally Posted by jyasaki View Post
For the speedo question, there should be a very tiny hole in the leading edge of the lower unit. That will be connected to a small plastic or rubber tube that will exit at the top of the lower unit. Your speedo tube will connect there (should you wish - more on this in a bit.)

As the motor moves through the water, you get an increase in pressure within the tube, and that's what the speedo reads. More pressure, more speed.

Problem comes from one of 2 things: Either the tiny hole gets clogged (and it doesn't take much to stop it up), yielding either a false reading (worst I've seen was 18mph while sitting on the trailer) or no reading at all, or the hose pops off the speedo, creating a little tiny firefighting unit within your console.
Is it usually just a straight through hole up to the flexi hose? Mine is totally blocked up and the speedo has about a 10 minute lag time before it catches up. I took the hose off the speedo end and blew 100psi down it from an airline but it still didn't clear it, just got slow drips of water out. Would it be safe to clear it using a small drill bit the right size for the hole (just turning it with fingers)
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Old 07 December 2006, 08:39   #7
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tommy

we might be able to help each other, i am no more than 20mins from you, i also have a yam 90hp 2 stroke on a 5.4 searider, my alum prop is the standard 13 3/4 with 17 pitch i am looking for a stainless steel prop and intend to try a 19 pitch for comparisons sake

do you fancy a trip down to the coast we might be able to swap props and have a day out. safety in numbers
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Old 07 December 2006, 08:45   #8
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tommy

forgot to answer your second question, the pitot tube comes out of the top of the lower leg on the transom side, it should be able to push on a length of pitot , and then push the other end on to your speed guage at the back,

I had probs with mine and took the lower leg off (only 5 bolts ) it was blocked easy enoth to clear.

ian
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Old 07 December 2006, 12:10   #9
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You can also take a tiny drill bit (one of those piano wire jewelers bits) and ream out the pitot hole in the front of the LU. It has a tendency of clogging with bits of shells, kelp, or just crytallized salt. The flutes on the bit will help avoid pushing all teh crap further into the tube (which actually won't be an issue if you do, but just isn't proper, I guess.)

jky
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Old 07 December 2006, 16:30   #10
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Thanks for all your replies and help. Where do i go to get a new prop off the shelf? Ian, that would be great to go down the coast with you and do some testing. I was looking at buying a 22p prop so between us we would have a good selection to play with. Blimey we might even be experts after! or not!!
If you wanna give me a call then i can be contacted on 07740948899 during the day to organise a time. If not then i'll just wait for your message back.
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