To prop or not to prop?

[Simon B]

Esteemed fora members can you settle this problem? A long running frank exchange of view has developed in our dive club as regards one of the boats. It is approximately 1000RPM down in normal operating conditions. It never revs over 5000 and is a 4 stroke, WOT in manual is I think 5000 to 6000

How important is this and should I be pushing to get it sorted out we've only ever had 6000 out of her with just 2 people on board. If not sorted what would be the potential problems in terms of safety, clearly we are throwing money away on fuel but I'm overly concerned about that yet.

[Pete7]

Simon, its a trade off between getting the best performance by changing the prop against club funds. But 1000 reves is alot. Sounds like it was propped oringinally for 2 people but when its full of divers and lots of heavy equipment it must be really struggling.

We dropped 3" on our dive club boat so the boat will accelerate properly with 6 divers and kit.

Pete

[Rupert Bear]

You have a problem with a dive boat.
The two conditions it runs in are very different so props will always be a compromise.
If you prop it for loaded condition you get the best performance most of the time but because it is a club boat you always get some yahoo who will over rev it when empty (and it is empty quite a lot when divers are in the water) Some folk either don't know or care what they are doing with it as it doesn't belong to them.
If you prop it for empty you lose performance when its full (again, a lot of the time).
It depends on whether you think it will be abused which option you choose. I would probably prop it for full and put a bit sticker or similiar saying max revs on the rev counter with a boat rule threatening dire consequences to folk that ignore it

[Jackwabbit]

I had the same argument with my club; the boat was supplied with a 13 ¼” X 17” prop, it would do 32knts with only 2 people onboard and rev to 5600RPM. With 6 divers it would only rev to 4500RPM, & it would take forever to get on the plane & once it was it would only do 25knts.
In the end I went out a got a 13 ½” X 15” prop out of my own money just to prove the point. It now gets on the plane quicker & the engine now revs to 5000RPM with 6 divers on board, while still doing 25knts. This proved the point & the club gave me the money back, but I’m now thinking to get the best performance I should have gone for the 13 ¾” X 13”. Speed with a small rib full of diving gear is not everything, but to have the power on tap instantly is far better in my opinion.
Push to get it sorted they will thank you for it in the end.

Almost forgot to mention the other benefit.
The quicker the boat gets on the plane the more fuel you save.

[jwalker]

Sounds like you need a Torque shift.

[Simon B]

Wassaht JW? Modular prop?

Another quaint habit from the club is not carrying a spare how many fora members carry a spare prop. If we re-prop with the correct one then we'll have a spare...Oh decisions decisions!

Jackwabbit, am tempted to follow your lead.

[jwalker]

I don't know whether they're available for your engine but they will solve the problem.
http://www.land-and-sea.com/marine/t...rque-shift.htm

[Rupert Bear]

I was lucky and the Yam plastic modular version came with the engine. This breaks down into 6 bits and fits under the seat.