Quick prop question

[jamie1131]

I run a 17P Hi Five prop, getting onto the plane is easy no matter how many I have on board but top SOG is about 26-28Kts and I'm pretty sure that the boat/engine combo should be capable of a lot more, the guy that had the boat before me used it for towing skiers so that might explain it!
I'm planning on trying a higher pitch, as accelleration is very good I can afford to sacrifice it a bit in search of a higher top end, on calm days I'd like to be able to cross the solent a bit quicker!

[Brit Rider]

Will get that high 5 out to you this week for testing and then report back here

Cheers,

Mike

[benbaron]

Turns out it wasnt a quick question


Thanks Mike

Thanks for all the rest of the input guys - much to digest....glad I didnt plumb for the 21p on evilbay then

[benbaron]

Is it possible to change the prop whilst the boat is in the water? Im just thinking Id like to do a run on my current prop, change it then run again - best comparison - I have a habit of forgetting how things are you see....


Next question....changing the prop - hows that accomplished? and are there any pitfalls I need to know about?

AND how would I know if its spinning the prop?


Cheers all

Ben

[NikTheGeek]

Is it possible to change the prop whilst the boat is in the water? Im just thinking Id like to do a run on my current prop, change it then run again - best comparison - I have a habit of forgetting how things are you see....


Next question....changing the prop - hows that accomplished? and are there any pitfalls I need to know about?

AND how would I know if its spinning the prop?


Cheers all

Ben

You *can* change it in the water, but I'd make sure it was very shallow water coz as well as losing the castle washer, nut and prop wrench, you could drop the prop in too! That would be an expensive mishap

You are supposed to change the washer and nut too but TBH I only change them every few prop changes.

[benbaron]

Cheers Nik...might be wise to not do it that way then

[salamis]

In reality it isn't that easy to swop over props as the boat needs to be pulled out to change them over plus the trek down river and picking a day when you have the time to do it all.
It is done of course but usually you have to settle for taking note of speed and engine revs then next time out use other prop and note similar for a comarison later.

[Centaur]

As Nik says, you can change the prop when afloat but it is not an easy job - it would certainly be better to have a dry run before attempting it on the water.

Once you know how all the bits go together though, it can be useful to be able to do it on the water - one day you might have to! Bear in mind that you will have to perform the task somewhat blind, as the prop is almost certainly going to be under water! However, if you have got one or two hefty mates that you can put up in the bow, you might get it above water level. Note also that an extended swim platform can make it very difficult to reach the prop. The trick to not losing any of the bits (you will drop them!) is to put a large mesh bag around the outdrive and always carry a spare everything.

Lastly, you can get plastic floating prop wrenches too but don't bother... it won't help you if you drop any of the other bits and, if the retaining nut is very tight, it will just strip the soft thread on the wrench. Carry a metal wrench and a mesh bag!


Oh, and when you say 'how would I know if it's spinning the prop' I take it that you mean whether the prop is slipping on the hub? This tends to be an all or nothing problem - if the prop hits a submerged object, the hub is supposed to shatter to save the drive (but not the prop itself). If this happens you will need your spare prop and hub and your new-found ability to change the prop on the water! If you are concerned though, you can inspect the hub for damage / wear when you remove the prop. However, note that there will always be a bit of prop slip through the water itself as the prop is not 100% efficient. This is perfectly normal and, for a sportsboat, prop slip will probably be in the order of 85%-90%. You can determine the amount of prop slip by calculation but you must first accurately establish the boat speed and revs at WOT, check what pitch prop you are running and find what reduction ratio the sterndrive is (it will be stamped on the gear case). From this data you can calculate both the theoretical and actual forward motion of the boat per revolution of the propeller - the ratio of the two is the prop slip.

[Capn Jack]

ok Ive ordered some bungs for the prop first off

Good idea, although the Laser 11 is primarily a top end lightweight performance prop for lighter boats with engines from 135 - 200hp. It should also give very good hole shot because of the PVS (performance venting system).

Depending on the power/torque curve of the engine matched to a boat, will depend on whether a certain prop will perform the way the skipper wants it to. Certain props like the Laser 11 will perform well for one skipper but not for the other.

The Laser 11 does have a large blade area, whereas a High Five has smaller blades. Why?

If a High Five of 19" pitch had each blade the same area as a Laser 11 19" pitch blade, then you are in principle adding two extra blades, so you would have to go down to a 15" to get the same type of performance, a 5 blade prop will also be smoother but less efficient so you would lose the benefit.

After all this tech stuff, things become a bit fuzzy, they don't actually tell anyone what will happen when a new prop is fitted to a boat. Therefore when you get an offer to try one, it's Christmas.

[Overdraught]

To follow on from Centaur's theoretical max speed from a given prop;

My 17' bowrider with 3l Mercruiser has a 2:1 Alpha drive and, according to the manufacturer, weighs 1030kg dry, so with the standard 21" prop could achieve 4800 (max. rpm) X 60 (mins/hr) x 21 (inches) per hr = 6,048,000 inches/hr. There are 1760 yards in a mile and 36 inches in a yard, so if i divide 6,048,000 by (1760x36), i get 95.5mph. Gearbox ratio is 2:1, so that's a bit under 48mph. Actual top speed lightly laden is around 44, so prop efficiency is about 90%.

Have i got that right?

To get back to the real world, with a 19" four blade ali 'Turning point Hustler' prop, my top speed lightly laden drops to 38mph. However, with 'Smarttabs' in the down position, 5 persons and about 70kg of gear on board (total load about 450kg), it will still pull me up on a monoski with both feet in the bindings from a deepwater start. It takes quite a few seconds, but this is considerably more difficult for a boat than popping a wakeboarder up, and a big saving on an Aerostar prop too!

Overdraught

[jamie1131]

Not sure if anyone has mentioned this, but for prop calcs and notes you need speed through the water, not speed over ground.

[Capn Jack]

Overdraught, I concur. Speed would be 47.725mph. However as jamie said its through water, but the difference would be small.

90% efficiency is outstanding.

The 19" four blade is like putting a car into 1st instead of 3rd.

[T 15]

found this today, while looking for a "new" s/s prop for T15......

http://www.google.co.uk/url?sa=t&source ... qTRLjFhr7w

may be some help, also give engines rpm's etc