Advice needed

Bigj · 05 May 2008, 19:41

Can anyone help out with a problem I have been experiencing with my Suzuki DT65 longshaft? I had it fitted to my Chinook 5m RIB last year and since then I have experienced a very annoying problem. If I apply power the boat picks up speed without any problem until it reaches 20-26 knots (sorry I dont have a tacho fitted but not at full revs). When it gets up there it then slows quickly and the engine revs increase very quickly almost as if you were driving a car at speed and then knocked it into neutral with your foot still on the pedal.

The prop is a 16" pitch and 10.5" diameter. I swapped it over today thinking that the inner bush may have gone but the problem is still there. I've also tried different angles of tilt without success. Is this cavitation? I have anti-cavitation fins fitted. If so how do I cure it? I managed to get the boat up to 26 knots today on a flat sea before it kicked in and brought me to a stop but it felt like there was more speed available if the problem wasn't present. On slightly rougher water (following in a vessels wake) it kicked in earlier and I was forced down to 13 knots to get back to port.

Hope I've described it properly, still new to boating.....any advice would be appreciated.

Joe

captnjack · 05 May 2008, 22:07

I'm guessing the gearbox is worn or misadjusted. Other than the prop bushing that's the next mostly likely place for such dramatic slippage. Happening sooner under load is consistent with that. I doubt its cativation, that would not slow you to a crawl. Is the motor new?

Observer · 05 May 2008, 22:08

That's cavitation (or, I think more correctly, aeration). Basically, the prop is 'slipping' through the water instead of biting and providing drive.

I think (someone may correct me) you need a larger pitch prop.

captnjack · 05 May 2008, 23:30

Quote:

Originally Posted by Observer

I think (someone may correct me) you need a larger pitch prop.

That is about the largest diameter prop which fits that lower unit. And actually fairly steep in pitch (17" being the max available). If the boat is actually dramatically slowing you have non-prop problems. As far as I can tell that engine must be at least 10yrs old. I still think you have a gear problem.

Bigj · 06 May 2008, 09:08

Gearbox! Bloody 'ell that's all I need! Bought it from a reputable dealer so would be disappointed if it was as its hardly been used since being fitted. If its any use the gearbox engages and disengages smoothly and there are no clunks or rattles at all from it. When it happens it really does bring you to an abrupt stop in the water with the engine reving away but as soon as I back off and reapply the power it picks up again instantly.

Is there a check that I could do to identify a gearbox fault apart from stripping it down?

bedajim · 06 May 2008, 09:23

Quote:

Originally Posted by Bigj

Gearbox! Bloody 'ell that's all I need! Bought it from a reputable dealer so would be disappointed if it was as its hardly been used since being fitted. If its any use the gearbox engages and disengages smoothly and there are no clunks or rattles at all from it. When it happens it really does bring you to an abrupt stop in the water with the engine reving away but as soon as I back off and reapply the power it picks up again instantly.

Is there a check that I could do to identify a gearbox fault apart from stripping it down?

Have a chat to the dealer and do you have a picture of the transom/engine

Bigj · 07 May 2008, 19:50

Finally managed to get to the bottom of this. The original prop had failed on the bearing and it seems that the spare which I kept off my old DT60 was also knackered hence the repeat of the problem when the props were swapped. I've now bit the bullet and bought a new prop which I will fit this week and hopefully cure the problem. The new prop is a 14" rather than the 16" which is fitted at the moment as the general consensus of opinion is that the 16" over props the boat and is a likely contributor to the excessive fuel consumption and poor planing that I have experienced.

With a new, smaller prop I am hoping that both performance and consumption will improve.

Bigj · 07 June 2008, 09:45

Update. Fitted the new prop and the problem is cured. The get up and go that had got up and gone is now back.

spartacus · 08 June 2008, 22:19

I'd double-check the gearbox oil to make sure the over-reving hasn't stripped anything. Oil should run clear, with no metal debris.

Downhilldai · 09 June 2008, 00:24

I've got a nice 16" pitch prop to suit a DT65 if you find that 14" too fine, or if you need a spare