Transom Weight Limits

[demondesmond]

Hopefully some of you experienced boaters (or even boat designers) can help me........I have a 5mtr Humber Destroyer which is rated at max of 90HP or 135kg outboard weight. According to the Humber website Rigid Inflatable Boats - a vast range of quality ribs for both leisure crafts and sports craft from Humber Inflatables the 5.3 is also rated at 90 HP but a max weight of 165kg. My question is why does the short increase in length (30cm) make such a big difference to outboard weight limit? To my untrained eye the other outboard weight figures and baggage weight limits accross the Desroyer range (at least between 5 and 6 mtrs) seem to be somewhat eratic. Can anyone explain to me how these limits are worked out?

Thanks

[demondesmond]

Is it really such a dark art or am I being extremly thick? Can nobody offer an explaination?

[9D280]

This isn't going to be a very teccy answer, but it basically boils down to the structure and balance (both at rest & when on the plane) of the boat. For example mine is a mid 80s version of what is now the O-Pro. the transom is 50mm thick (including the fibreglass) and has no solid bracing. Rated 60Hp / 110Kg. The current 5m O-Pro takes 75 gee gees & 140 odd KG (I forget the exact number).

As the boats get longer they will also get heavier. To beef up the transom on the 5m to take a heavier (bigger) engine will likely mean 3 things - 1) it will be so tail heavy it will handle like a pregnant whale. 2) when you stop, you get flooded. 3) you end up being able to hang waaaay too much Hp on the back for a 5m hull.

Your extra 0.3m may be "out the back" - i.e the cones protude further, thus giving more bouyancy so more weight can be added as there's extra toob to keep them afloat?

I'll have to had you over the he Humber experts for the fine details of that hull range tho'.

[Markyboyo]

I remember being equally confused when I bought mine.

I was looking at a 5.3 but decided on a 5.5 as an extra 20cm meant I could have 25 more HP on the back.

Looking at the figures, despite being allowed a 115 on the 5.5 instead of a 90 on the 5.3 you are only allowed 5kg more on the transom actually making me 3kgs over with the E-tec 115.

I'm not sure you can even get a new 115 under 170kg, especially with the XL Transom I have.

[demondesmond]

Now then. There are a few things there that I hadn't thought about. I had assumed that the construction of the transom / hull would be exactly the same for all Destroyers just slightly longer. Also, the length of the sponsons from transom backwards is interesting.
My problem is that I'd like to put a small amount more HP on the back but finding a new donk to replace my 60HP ETEC within the weight limit is a nightmare. Yam F70 is an option but there is an additional weight penalty with this and I'm not sure I get he extra 5 / 6 Knots top end I'm after. I've been trying to find someone with 5m Destroyer + Yam F70 (another thread) but no luck yet. I can't understand how the boat can be rated at 90HP when there are no current 90HP engines less that 135Kgs. I have spoken to Humber about this but their answer was "it is rated at what it is rated at". I should have gone for 5.3 or above but storage was the issue.

Thanks 9D280

[demondesmond]

Thanks Markyboyo
Sounds like you'll have some grunt there. If you are slightly tail heavy whats the handling like on your boat? Does it washover when you drop off he plane and stop? Can you trim the engne in far enough to keep the nose down on take off?

Des

[Markyboyo]

I think it is a little stern heavy but once you are up and running the bow comes down nicely and it handles fine. It does come up if not trimmed right in but it gets onto the plane fast enough for this not to be a problem even if 6 up.

If I could go back I would probably have had a 5.3 with a 90 E-tec as I could maybe have sneaked that onto an un-braked trailer.

I don't have any issues with wash coming over the transom when stopping but then I did specify an XL transom when I bought the boat which raises it by 5". I'd used several ribs before that were prone to this hence my choice.

I'd be tempted to throw caution to the wind and stick a 90 TLDI or E-tec on yours as soon as the warranty is up!

[demondesmond]

Markyboyo... look at you with your caution to the wind! 90 Etec is an option that I have been thinking about for some time (albeit more power than I think I need but I'm sure the boat will handle it) and I think that is the most likley outcome. I have the long shaft transom but as you can see from the attached pic she sits pretty high at rest next to my mates One Design with 115Etec. He does get wash over the back if not careful dropping off the plane. I might have to lose the A frame though.
With the Yam F70 I was trying to see if I could solve the problem by sticking to Humbers weight limt.

Thanks for your comments

Des

[Markyboyo]

I see where you are coming from with respect to staying within Humber's weight limit but for what it would cost to upgrade from your 60 it would be expensive for an extra 10 horses.

That One design does look very low in the water, I wonder how different your destroyer would look if you chopped off the raised splash plates down to the same level as the centre of the transom like the one design? I also reckon that the tubes on the one design hull are mounted slightly lower on the hull relative to the centre of the transom?

My Aframe is a double one and I was very surprised at how light it was when I fitted it. It bolts to the inside of the transom and also to the floor a foot or so forward so I hope it provides more strength to the transom than its weight takes away.

I'd love to see how yours handles with that little weight on the transom, I bet you can really throw it around. Let me know if ever you decide to do some ribbing in Cornwall!

[PeterM]

Originally Posted by demondesmond

He does get wash over the back if not careful dropping off the plane.
Des

That'll be why he has water coming out the bilge pump in that pic on flat calm day ?

[9D280]

Originally Posted by Markyboyo

I'm not sure you can even get a new 115 under 170kg, especially with the XL Transom I have.

Aye, and that's half the reason I now have a Clamshell..... 110Kg rated transom, 85Kg engine. I've even got capacity for an aux! (which, incidentally, is a 13Kg (fuelled) 4Hp of 1974 vintage. How heavy is a 4Hp these days?) My entire setup clocks in at 420Kg fuelled.


Another Q that I should have asked before - how old are the boats you are talking about? I'll take my transom as an example - 110Kg rated. Not much I can actually hang on that at 60Hp, even in the 2- stroke arena. But then I look at all the old broures of the late 70s / early 80s. Most engines of that size were 2 cyl premix jobs, and power trim (another few Kg) was a very expensive optional extra if it was even offered at that size. 110Kg capacity was well more than enough. Not much later after my hull was built everything was 3 cyl and had autolube. My hull was redesigned slightly & renamed the Attaque. Transom now rated to 130 odd KG to handle all the extra weight....

Now, drag that observation into 2012. Sometime about 10 years ago a similar problem was had when these "lardy 4- strokes" appeared - do a search on here for 4 strokes & searider SR4s.

[Markyboyo]

My hull is a 2008 and I think Desmonds's is probably newer or about the same by the looks of it. I have a nice light aux too, 3.3 Evinrude of 1998 vintage which weighs about the same as yours but I keep it in the boat rather then hanging it out back all the time. Not really powerful enough but does the job and I don't want anything heavier.

I used to have a 1986 Orkney fishing boat with a Jap built Mariner 40 twin on the back. I think that engine weighed about 65 kgs, I had a bit of a shock to see the weights of newer 40's when I replaced it in 2006. Went for a Tohatsu 50 as it was as light as I could get new but was still over weight for the transom. That was a 20 year old boat though.
It just seems silly that Humber's current engine weight and HP ratings for some models can only be achieved by fitting a 25 year old engine but who fits an engine that old to a brand new rib fresh out the factory?

[demondesmond]

My Destroyer was built late 2010 I picked it up early 2011. It was a turnkey package from Humber and although I am on here asking about more power / transom weights etc I have been broadly happy with the package (give or take a few fixture and fitting repairs). I'd initially asked for 30Kts so it arrived with a 17 pitch prop which struggled to plane / stay on the plane at low revs, didn't get to the recommended WOT revs but hit 31Kts. I replaced the prop with a 15 pitch and this transformed the boat so it planes and holds great now and is bang on the 5750rpm WOT. The down side to this was a reduction in top end to about 29Kts. The reason that I think the boat will take 90HP is you can hurl it around (not when Mrs Desmond is on board though!) even is some fairly choppy conditions and it feels sooo stable. I think I might load up the transom with another 35Kg to see if it upsets the balance before I take the plunge.
Oh by the way my Etec 60HP is 109Kgs supposedly.
The One Design is a cracking boat for speed and it can catch some big air but as we launch from Knott End over the side of the slipway he does get use out of his bailing bucket. The day of that pic was a great flat calm day.

Des

[demondesmond]

Markyboyo If we're ever down your way I'll give you a shout. Always up for a bit of exploring. We tend to go the other way in the summer onto the Mull of Galloway but hope to get further up the west coast over the next couple of years.

Des

[Erin]

I would imagine some of these power and weight ratings are a legacy of old designs, even though engine types and weights have changed since. I doubt manufacturers are going to have old designs reassessed just for the sake of meeting the demands of modern outboards and a handful of people that actually bother to comply with the small print. Don't get me wrong, I think it is very important not to breach the recommendations as you could land youself in trouble if you have a problem and need to claim off insurance or prove you weren't negligent. If the hull design and loadings were designed and approved back in the day, then there would be a reasonable cost to get it updated. I'm sure a lot of it was just sensible guess work at the time, but as that is what was written into the type approval (for want of a better phrase) that is what you will have to live with. Particularly as RCD now comes into it.

[demondesmond]

Sorry not familiar with RCD?

[Erin]

Recreational Craft Directive. You will have heard of the category A, B or C rating dependant upon what sea types and how far offshore a craft is designed for under EU rulings. Came in in 1998 I think. Cat B or over requires drop testing of a hull which I believe results in destruction and is therefore something smaller companies are reluctant to do too often.

[demondesmond]

Arrrh.......all becomes clear. Great forum for picking up stuff. Big toobs solid boat for a 5m (maybe)

[horacebat]

The ribcrafts did quite well out of this:

Boat Drop Test - YouTube

[demondesmond]

Cheers horacebat. I have seen something like this before as part of the Zodiac sales speel when we bought our previous boat (which is for sale incidentally see Ribs For Sale section). I seem to remember they whacked the boat against the quayside whilst it was dangling from a crane. We shortlisted the Ribcrafts and the Humbers for our present boat and it came down to geography in the end with Humber being closer to us. I know the Ribcrafts are considered to be some of the best small Hulls though.