Unusual RIB - Opinions

[Smithy_843]

Hi all!

I’ve viewed a RIB and completed two sea trials but something doesn’t quite seem right. Also a bit of a marmite boat so thought I’d try and get some opinions.

The boat is an ex superyatch tender. It’s 9m, stepped cougar hull (no HIN present), volvo penta TAMD63P (370hp), ZF286A gearbox (1:4.8 ratio) with a surface drive and trim tabs. I saw stepped hull, 370hp and surface drive an expected the thing to be pretty quick……..but it’s not.

On first sea trial it wasn’t so noticeable, the owner took it fairly careful and then slowly took it up to 28 knots before backing off and coming back. I did think it was reving fairly high at low speeds and acceleration seemed slow but didn’t know what the normal rpm was for the engine. Anyway, a quick google and turns out 2,600rpm is cruise and 2,800rpm is top end so I was prepared to pay attention second time around.

On second sea trial (I took a friend) it really felt like something wasn’t right. The owner had the throttle pinned, 2,800rpm and we were doing 8 knots going with the tide??? I asked him what was going on and he said he needed to use the trim tabs to get it on the plane, which he then did and managed to get the boat upto 28 knots at circa 2,500rpm (although it did seem to take awhile to get there). At low speeds it seems to have very poor acceleration, I mean terrible, but it does eventually get up to speed. The owner claims it will cruise at 30 knots and top out at 38 knots (I suspect this is ambitious based on what I’ve experienced).

A little background, the boat originally had a ZF300 2 speed 1:4:1 ratio gearbox and this was swapped out for a single speed 1:4 ratio box when the original box failed.

So my thoughts are:

- What the hell is going on with the performance? Could the change in gearbox ratios had a major effect? Or could the prop need replacing? It’s a fairly heavy boat with teak decks and a big engine but the engine should be powerful enough to give it some good acceleration and top end.

- Engine runs very well, starts easily, small puff of smoke on start up but then generally smokeless. But it is a big old lump for a 9m boat, and therefore quite noisy. Is engine bay sound deadening possible on inboard RIBs or would the benefit be marginal? At low speeds it felt a bit commercial, like a tug boat….

- Smell wasn’t too noticeable until the wind shifted behind us then I got a few smells. Is this typical on an inboard diesel RIB? The exhausts aren’t submerged, if they were would this likely improve?

Anyway, I’ll upload a few images so you can let me know what you think.

Cheers,
Ben

P.s. I’m fairly new to this, owned a Camel Trophy RIB around 15 years ago but that’s my only real experience of boating outside of sailing.

[Iankristy]

A couple of things that jump straight out to me are:
-how much? That’ll affect risk/ reward
- V looks pretty shallow wouldn’t like to be in challenging conditions with that
-helming a boat with surface drive/ trim tabs is a skill does the current owner have that? Full revs at 8 knts suggest he has no clue
-with respect a camel 15 years ago to this is a big jump.
- I get the luxury thing but…:Would you rather have a few friend shitting themselves in this in a decent sized sea or screaming with delight in a slightly more utilitarian but built to perform rib?

Depending what they’re asking for plenty ribs would run circles round this thing !

[Smithy_843]

Originally Posted by Iankristy

A couple of things that jump straight out to me are:
-how much? That’ll affect risk/ reward
- V looks pretty shallow wouldn’t like to be in challenging conditions with that
-helming a boat with surface drive/ trim tabs is a skill does the current owner have that? Full revs at 8 knts suggest he has no clue
-with respect a camel 15 years ago to this is a big jump.
- I get the luxury thing but…:Would you rather have a few friend shitting themselves in this in a decent sized sea or screaming with delight in a slightly more utilitarian but built to perform rib?

Depending what they’re asking for plenty ribs would run circles round this thing !

Boats up for £29k, could probably land it for £25k. Needs a retrim and the gel coat needs some work (it’s a 2006 boat). To be honest I’m not desperate to get back on to the water, the boats I’ve been keeping an eye on are typically 26-28ft cuddy cabin boats (Rinker 268, ect) this just came up and seemed like some cheap fun for the summer - I have two boys who would love me to get a boat. I’d mainly use it for traveling from Southampton to local waters (Isle of Wight), ancher down and sunbath/use some water toys (paddle boards, ect). I’m not looking to tackle rough water necessarily.

[Smithy_843]

Originally Posted by Iankristy

A couple of things that jump straight out to me are:
-how much? That’ll affect risk/ reward
- V looks pretty shallow wouldn’t like to be in challenging conditions with that
-helming a boat with surface drive/ trim tabs is a skill does the current owner have that? Full revs at 8 knts suggest he has no clue
-with respect a camel 15 years ago to this is a big jump.
- I get the luxury thing but…:Would you rather have a few friend shitting themselves in this in a decent sized sea or screaming with delight in a slightly more utilitarian but built to perform rib?

Depending what they’re asking for plenty ribs would run circles round this thing !

Sorry, to answer your other questions:

- shall v does concern me somewhat. I have friends that have boats we regularly go out on, so I’m not a complete novice but equally it’s been a long time since I’ve owned a boat

- current owner has had the boat for 12 years, you’d think he’d know what he was doing but practice tells me he possibly doesn’t!!

- it is a big jump, and it’s been long enough that I’m probably full amateur at this point. It’s not a small boat to be jumping into

I think it’s probably not the boat, it looks great, but I think it’s too compromised. I kind of know it, just needed someone else to tell me!

[spartacus]

Current owner might have had it 12 years, but it could have been dry-stacked for 11 of them.

With these things, and I know nowt about diesel ribs, then if you're test driving you're going to throw caution to the wind and blow the cobwebs away. Like I said, it's a test drive.

In my experience sellers should be happy to talk and have more paperwork than a HMRC dawn raid. If the whole episode was pedestrian, then walk away.

[paulbrown22]

Performance certainly doesn't seem quite right/at it's full potential for that hull/engine/drive combo. My old 9m Ribtec with a Yanmar 315 running a Bravo 3 outdrive would hit mid/high thirties. There might be a bit of turbo lag fairly common to diesel RIB's of this vintage, but that aside, if the gearing is correct, I'd expect more than 28kn, especially given the stepped hull and surface drive!

[Smithy_843]

Originally Posted by spartacus

Current owner might have had it 12 years, but it could have been dry-stacked for 11 of them.

With these things, and I know nowt about diesel ribs, then if you're test driving you're going to throw caution to the wind and blow the cobwebs away. Like I said, it's a test drive.

In my experience sellers should be happy to talk and have more paperwork than a HMRC dawn raid. If the whole episode was pedestrian, then walk away.

Thanks. He does have it dry stacked, and recons he’s used it for less that 200 hours in 12 years.

I did think it was weird that he wasn’t trying to push top speeds in it, like surely he’d want to show me what the thing could do? Instead it was all very PG and cautious.

Such a shame, aesthetically it’s a great looking boat and the layout would really work for me. It was on the pontoon next to a new 9m stingher and it really holds it own against boats worth 4-5 times more……..until you start it up……..….

Back on boat search I think

[Smithy_843]

Yer it’s really strange. The acceleration is the thing that got me, you pin the throttle and it literally doesn’t move, almost like the prop has zero bite, it just gradually grains momentum as the trims are adjusted and it gets on the plane.

Very naively 370hp and surface drives had me thinking this was a 40+ knot boat. I’m my head I was thinking I could justify the lack of manoeuvrability if it was quick, I.e. it’s compromised but for a reason.

[beamishken]

The 2 speed gearbox would be the clincher for this boat the engine doesnt have the rpm range to perform properly with a fixed drive. Most diesel ribs & fast sports boats use engines that rev out close to 4krpm without the 2 speed box to take advantage of the torque of the engine this thing isnt going to perform no matter what you do.
Without the 2 speed box I'd imagine its always going to be a bit of a dog.
Imho

Sent from my SM-G950F using RIB Net mobile app

[Limecc]

Six blade and non-duo prop?
Sounds like there's a lot of slip going on.

The D6 350hp version is rated between 47-60kn.
New Volvo Penta D6-350 and D6-370 models: Diesel engines with real high-speed potential

I'd be wary of older Penta engines there's currently some controversy over lack of ECU's and non-support. Apparently they fail (eg using with a dodgy battery) rendering the whole boat useless without a re-power. Max rpm is 3500 I think.
Volvo TAMD 73/74 and 75 ECU - Cause for concern?

[GuyC]

It's a very interesting propulsion set up, which I know very little about but as this is a forum, it doesn't stop me having an opinion.

At rest the surface drive is very definitely sub surface. I guess the trim tabs try and get the prop near the water surface so it can start operating a bit more efficiently.

The low top speed is possibly because the gear ratio doesn't allow the prop to spin fast enough and the boat is possibly too stern heavy to get the surface drive out of the water. Looking at the pictures, there doesn't appear to be much room to lift the surface drive up.

I think I'd let this one go although it would be pretty cool to get it going as the drive and the stepped hull would point to pretty good performance.

[Limecc]

Originally Posted by GuyC

The low top speed is possibly because the gear ratio doesn't allow the prop to spin fast enough and the boat is possibly too stern heavy to get the surface drive out of the water. Looking at the pictures, there doesn't appear to be much room to lift the surface drive up.

Originally Posted by Smithy_843

..almost like the prop has zero bite

2800rpm@8kn? Definitely spinning fast enough. Slipping. Ventilation?

I wonder how much the engine would fetch with that rarer-than-hen's-teeth working ECU? Could be a tidy penny. As a project for someone with the right resources a DF350 Suzuki outboard would drive it quite well.

[spartacus]

Originally Posted by Smithy_843

Thanks. He does have it dry stacked, and recons he’s used it for less that 200 hours in 12 years.

That works out at 16.6 hours a year. Okay we had lock-down for one of those years, but that tells you all you need to know. Shame - as nice looking boat, but sounds like it needs overhaul in order to get the best from it, and that ain't going to be cheap.

[Limecc]

Somebody spent a lot of money initially buying the tender and was disappointed when the drive failed. Was poorly advised or no like-for-like replacement available so spent another small fortune getting it seaworthy again, but it's not for for purpose.

Current owner needs to get a reality check. Unless he finds a naive buyer or takes a huge hit he's stuck with it and the dry stack costs will continue mounting for a boat he doesn't want to use.

I wouldn't be overly concerned by any shallow V. It might be a little slammy but the hull designer gave it steps so it should be quite efficient for speed and not have too much draft for beach drop off's. That's what this RIB's about.

[Tim M]

Or current owner just doesn't know how to drive a boat with surface drives? If you've got no or little experience it not as easy as just slamming the throttle forward to get the max speed out of the boat; it takes a bit of experience particularly with that particular boat to get to know its sweet spots such as trim tab position etc. Too much trim tab will slow you down, too little and you won't get the back end up enough for the surface drive to be able to do its thing.

[Limecc]

Originally Posted by Tim M

Or current owner just doesn't know how to drive a boat with surface drives? If you've got no or little experience it not as easy as just slamming the throttle forward to get the max speed out of the boat; it takes a bit of experience particularly with that particular boat to get to know its sweet spots such as trim tab position etc. Too much trim tab will slow you down, too little and you won't get the back end up enough for the surface drive to be able to do its thing.

Who wants a boat like that? Crap.

[Searider]

I think that the issue is that it's a fixed surface drive - it doesn't look like it can be trimmed up or down.
At slow speed the propeller fully immersed and effectively there's twice as much of it in the water as there would be at speed - therefore a huge overload.
Normally this is alleviated to some extent by feeding air into the space forward of the top half of the propeller and/or trimming the drive right up to let the propeller slip, the engine and turbo to spool up. You can then balance the trim against the engine speed.
As the drive can't be trimmed up it sounds like the boat originally had a 2 speed gearbox to allow the engine to develop maximum power / RPM to get the boat on the plane. Once there you change up to 2nd gear.
I reckon that an Owner didn't like the cost of a replacement 2 speed box!
The 2 speed boxes are typically used in Sunseeker superhawk 40s and 50s with fixed Trimax drives.

By the way I saw the ad pop up somewhere last week and thought it could be quite interesting IF it did 38 knots and cruised at 30!

[Limecc]

Originally Posted by Searider

I think that the issue is that it's a fixed surface drive - it doesn't look like it can be trimmed up or down.
At slow speed the propeller fully immersed and effectively there's twice as much of it in the water as there would be at speed - therefore a huge overload.
Normally this is alleviated to some extent by feeding air into the space forward of the top half of the propeller and/or trimming the drive right up to let the propeller slip, the engine and turbo to spool up. You can then balance the trim against the engine speed.
As the drive can't be trimmed up it sounds like the boat originally had a 2 speed gearbox to allow the engine to develop maximum power / RPM to get the boat on the plane. Once there you change up to 2nd gear.
I reckon that an Owner didn't like the cost of a replacement 2 speed box!
The 2 speed boxes are typically used in Sunseeker superhawk 40s and 50s with fixed Trimax drives.

By the way I saw the ad pop up somewhere last week and thought it could be quite interesting IF it did 38 knots and cruised at 30!

Admittedly I know nothing about surface drives but it's now a fixed gear and operating in the WoT range. I'm not buying that it's bogging down.

[GuyC]

In post 1, he suggests the gearbox has a 1:4.8 ratio rather than a 1:1.4 hence why I didn't think it was spinning fast enough.

At low speed, the prop is just slipping but once you've overcome that hurdle by I guess trim tabs and gentle application of the throttle, the prop just doesn't turn fast enough to get the speed to give it enough lift to get the surface drive to the surface.

I couldn't tell whether the large triangular plate was fixed or had a hinge thing going on.

[GuyC]

I had a look at Dave Gerr's propeller handbook and his comment is that you can't calculate surface drives in the same way as normal props.

My only suggestions to the OP would be to ballast the forward end of the boat to try and get the stern a bit higher to get the surface drive nearer the surface. Alternatively he could try and ventilate the prop with a bit of hose set forward of the prop.

Ultimately I suspect the gearing is wrong and it is missing a top gear which is what the two speed box gave.

https://www.boatsales.com.au/editori...ce-drive-8164/