Laden dive boat...prop change?

Razorbill RIBs · 16 May 2010, 20:00

We've been experimenting with loading up with lots of media /dive gear for a potential project we've got coming up. We'd need to carry a lot of spare dive gear for a series of dives.

The Ocean is a heavy old lump of a boat and not a fast hull by any means, but she achieves 40 knotts with the 150 Opti running a 19 prop.....more than enough for us!

Have run previously with heavy kit (media divers) and 6 up and whilst not spritely, have achieve around 27 knotts....happy with that too.....

For the project we would potentially need to be loaded up for 4 dives (Everyone diving twinsets) with 6 divers and 1 or poss. 2 crew.....so lots of kit, additional air etc....lots of big blokes too....we'd also be carrying additional fuel (over the 32 gallons in board).

Having loaded up with the same sort of weight we went for a trail run, but just couldn't get the boat to plane properly no matter where the weight was distributed and made around 12 knotts and max 3,500rpm.....time to rethink!

So, am i simply asking too much from the 150 Opti? OR might I have more joy if i try another (which?) prop specifically for when we are running with such heavy gear OR is there something else I could try....I don't want to run overweight /dangerously, but the sponsons were only just resting in the water at rest (usually float above), so confident the hull wasn't overloaded.....no need for great speed, but planeing would be nice

Not sure if it's relevant but the Transom is rated for max OB of 225....at this rate think we're going to need a second boat....

Any advice from divers /those in the know appreciated

Thanks,

Dan

Chris · 16 May 2010, 20:23

Dan, sounds like you need to drop at least 2" in pitch and probably 4". You are seriously overpropped for the load you are carrying which will f**k the Opti very quickly. Imagine trying to drive you car everywhere in 5th gear. See if you can try a 15" prop to start with.

willk · 16 May 2010, 20:25

Dan, I guess your rib has a max loading of circa a ton. That lading list sounds well in excess....

Bigmuz7 · 16 May 2010, 20:40

Thats a hell of a kit by the sounds of it .. on my 6.3 with an opti 150 we only ran at your similar speeds with 2 up .. with a full family and gear and a dingy strapped to the A frame I felt surprised as how quickly the power/speed dropped off ,.. but we ran a 21 for speed so a 19 would be normal for loads .. in your case maybe a 17 is the answer

or perhaps a four blader 17 which will be cupped less

chewy · 16 May 2010, 20:44

Does sound alot of weight, like willk said I'd find out what load it can take?

willk · 16 May 2010, 20:49

Quote:

Originally Posted by chewy

Does sound alot of weight, like willk said I'd find out what load it can take?

Having played around with the figures, I'm getting a potential load of 1500kg, which may be shiftable, prop-wise, but from an MCA point of view - tricky.

Bigmuz7 · 16 May 2010, 21:43

Quote:

Originally Posted by willk

Having played around with the figures, I'm getting a potential load of 1500kg, which may be shiftable, prop-wise, but from an MCA point of view - tricky.

Thats closer .. mine was about 1550 but it depends how you have it rigged

Razorbill RIBs · 17 May 2010, 07:37

Quote:

Originally Posted by Bigmuz7

Thats closer .. mine was about 1550 but it depends how you have it rigged

Bigmuz /Willk....I reckon your weight calcs are probably right at the 1500 1550 kg mark....I have been trying to find out what the carrying capacity for the Ocean is, but struggling so far....will give Malcolm Winsor a call and see if he can advise....

The main reason for testing is that we would want to re-do our stability tests (MCA) with all the kit on board....

Chris /Bigmuz, will try and get hold of /try a 17 prop anyway.....as even with less gear, the additional hole shot would be useful.....would swap props for 'laden' and 'normal' running.

I am fast reaching the view that a second boat is the answer....it's just making the numbers stack up.....

Thanks for your helps with this, will let you know how the 17 works when I get hold of one....Anyone got a cheap coded RIB for pennies?

Dan

GordyP · 17 May 2010, 11:07

Dan,

I use two sets of props on my rib, 21" for running around with only a few people on board and 17" for when we go out with 8 divers and full kit. I think the main problem you'll have from the sounds of things is just how busy the boat will be, getting kitted up won't be much fun.

Cheers.

Simon B · 17 May 2010, 16:05

Talk to the nice lady at Steel developments prop repair she have a reasonable idea of what's about right. Think also about a 4 or 5 blade prop, if you really want to push the boat out think about stainless (original post did not mention this) but cost may be factor.

I would have expected an engine of that size to do the job if you are not exceeding the wweight and have your prop correct. It will not be a racing machine but a guess would be 27-30 knots on flat water. Whether you would want to drive a boat at that speed with the weight you have mentioned is purely up to skill and experience - how are you off for insurance?

captnjack · 17 May 2010, 16:59

Once you're planing -2" typically = +400rpm

Since you only made 3500rpm (and WOT should be around 5500rpm) and couldn't plane I'd drop more than 2" of pitch. 17" is probably optimistic; I'd try a 15"

Tim M · 17 May 2010, 19:59

Ocean 6.5 is rated to carry 1000kg

willk · 17 May 2010, 20:02

Quote:

Originally Posted by willk

Dan, I guess your rib has a max loading of circa a ton.

Quote:

Originally Posted by Tim M

Ocean 6.5 is rated to carry 1000kg

is there a prize?

Razorbill RIBs · 17 May 2010, 20:13

Quote:

Originally Posted by willk

is there a prize?

Well done Willk!

...and thanks Tim (out of interest, where did you find that...have I missed something on the Ocean site?)

Captn Jack...take your point, will see if I can source a 15 to try...

Gordy, you are right, 6 divers is okay if not ideal, but space is going to be an issue with all that kit....given the weight issue, a second boat carrying gear is looking to be the best option.

Simonb...thanks for the recommendation /contact....will pick their brain too.

Thanks all....RIB net comes up with the goods again! Despite probably using a second boat, the smaller prop will be better for the boat when carrying 6 divers generally....will let you know what we end up with.

Dan.

wavelength · 17 May 2010, 20:43

6 divers, loadsa kit, extra air, and only a 6.5m boat - I really do think you are looking at a second boat even if only for safety.I don't know where you are going but if its somewhere where it might rough up while you are out you will be seriously struggling with a boat loaded like that (and I assume your tests have been made on flattish water). Its gonna be a whole lot harder to get it on the plane going uphill! and I really wouldn't like to be kitting up that crowded.

Simon B · 17 May 2010, 21:14

A second boat would be good, however, in good conditions (not many divers actually dive in bad conditions) it sounds do-able however, as your max manufacturers payload is 1000kg your insurance would be void if this is broken. We've run 8 divers and 2 crew on a 5.5 delta - cramped but all on single tanks and no ponys, not that teckie, sorry.

As wavelength said kitting up cramped is not good either, for diver, boat or crew. When our little boat did run loaded up it did not take much to knock her off the plane then you end up at full throttle climbing swell or waves that arent really that large with almost nothing in reserve except your own nerve.

Razorbill RIBs · 18 May 2010, 11:28

Spoke to the VERY helpful lady at Steel Developments

....after discussing the various weight scenarios and revs achieved etc, she has recommended a 15 pitch four blade.....one winging its way here now....thanks for the recommendation!

Will need a second boat for the additional gear /that project, but following my conversation with them the 15 sounds a good solution for when we are operating up to our 1 tonne max capacity (obviously excercising discipline not to over rev if not laden....)

Will let you know how it performs whe we get chance.

Thanks all

Dan

Bigmuz7 · 18 May 2010, 18:22

Quote:

Originally Posted by Dan Worth

she has recommended a 15 pitch four blade.....one winging its way here now....thanks for the recommendation!

Yeah I thought a 4 blader might help. Its certainly a heavy boat if the payload is only a tonne, you'll lose some more off top end speed switching to four though, IMHO and you will get more slippage from hole shot, but I found cornering grip improved at full bung, and the stern didnt drift as much, but that was on a fairly light boat,.. will be good to hear how you get on

Razorbill RIBs · 24 May 2010, 21:03

Quote:

Originally Posted by Bigmuz7

will be good to hear how you get on

To close this one out....have been trialling the new 15 pitch 4 blade prop with 8 up and weight.....hole shot great, top end reduced to 32 knots (more than enough laden...really happy at 25 knots)....biggest surprise is that even when we ran with just three on board, it still didn't rev past 5300rpm....max engine revs 5700rpm according to literature....cant get my head round this...not complaining, just thought it may /would have overrevved?

Really very pleased....thanks for all the assistance....big thumbs up to steel developments....recommended

Dan

chewy · 24 May 2010, 21:46

The revs could be to do with switching from 3 to 4 blades, you've got more surface area.