Transom weight capacity.

JohnW · 17 September 2019, 22:18

Could anyone advise if there’s a formula or calculation used to work out the maximum engine weight a transom can carry? (That’s weight in kg, not power in hp)
I’ve seen a few other people getting confused about stated outboard capacity in weight is nowhere near enough to allow the stated outboard power capacity, ie, I’m advised that our ribs capacity is 150hp but 130kg weight. I’ve yet to find a 150hp outboard weighing in at 130kg...

simonl · 17 September 2019, 23:02

There's a few formulae that could be used, but I doubt you'll ever get the info from the builder to input into them. I reckon it's either a case of getting stated capacity from the manufacturer or just seeing what others get away with long term - with the associated risks)
I have a small (no longer used) 15ft fibreglass boat with a 150hp V6 yam on it. It took some abuse over many years & never fell off. I imagine it was way overpowered. Freeboard was almost nil. Obviously much less of an issue with a RIB

JohnW · 18 September 2019, 13:57

Quote:

Originally Posted by simonl

There's a few formulae that could be used, but I doubt you'll ever get the info from the builder to input into them. I reckon it's either a case of getting stated capacity from the manufacturer or just seeing what others get away with long term - with the associated risks)
I have a small (no longer used) 15ft fibreglass boat with a 150hp V6 yam on it. It took some abuse over many years & never fell off. I imagine it was way overpowered. Freeboard was almost nil. Obviously much less of an issue with a RIB

Do you have the formulae?

simonl · 20 September 2019, 07:29

Quote:

Originally Posted by JohnW

Do you have the formulae?

I don't have a simple calculator & it's been some time since I've done any structural calcs. These links might give you an idea, or more likely will scare you off!

https://en.wikipedia.org/wiki/Young%27s_modulus
https://en.wikipedia.org/wiki/Poisson%27s_ratio
https://en.wikipedia.org/wiki/Finite_element_method

JohnW · 20 September 2019, 07:55

Yes, well I suppose I did ask didn’t I?��
I think I was hoping for something along the lines of if x thick and y long and z deep then it will carry x kg

simonl · 20 September 2019, 08:17

I would go to the local marina measure a good cross section, noting the capacity - or ask on here maybe if people could err... measure theirs!

Iankristy · 20 September 2019, 12:07

What kind of Rib is it? Sounds like the makers have already done the formula for you. They’re calculating the maximum forces your transom can safely withstand not the power to weight ratios of outboards. Best to fit one that satisfies both power and weight restriction. I know some insurers will allow you to exceed transom rating if you declare it but probs easier to accept the stated limits [emoji106]

JohnW · 20 September 2019, 12:45

Ribs a tornado. They say it can take 150hp max weight 130kg. We don’t want a 150hp, just a 110 or thereabouts. If you could point me to a 110hp outboard weighing a max of 130kg I’d be very grateful, but our current 90 weighs 139 so I think you may struggle.

Iankristy · 21 September 2019, 06:44

Quote:

Originally Posted by JohnW

Ribs a tornado. They say it can take 150hp max weight 130kg. We don’t want a 150hp, just a 110 or thereabouts. If you could point me to a 110hp outboard weighing a max of 130kg I’d be very grateful, but our current 90 weighs 139 so I think you may struggle.

Sounds like a good excuse to get a bigger boat [emoji3]

ribber · 21 September 2019, 08:19

Just had a similar quandary.
Transom plate should be marked up with what the hull was designed for.

I could only go up to 115 hp, so originally the boat had a Yamaha F100 @ 171kgs

I have now replaced that engine for a Suzuki F140A @ 179kgs

The Suzuki being 8kgs more than what I had on before.
I thought the back end would be too heavy and make the tubes sit in the water at rest, something I really wanted to avoid. Whether the extra torque will pull off the transom or not, will have to wait and see. Although we have added a horizontal stainless steel angle trim to spread the load over the top edge of the transom.

Interestingly, the Suzuki has an offset drive shaft and the weight is slightly moved forward over the transom, which helps very slightly to keep the back end higher.

My research being;
Old Yam F100 171kgs.
New Yam F100 162kgs
New Yam F115 174kgs
New Yam F130 174kgs
Suzuki DF100 182kgs
Suzuki DF115 182kgs
Suzuki DF140 179kgs

Weird that the larger Suzuki is lighter than the other two. Perhaps that means the block has been bored out to much and makes the engine weaker....who knows.

I have been playing around with the boat this month and can say the difference is amazing. Yes the engine may be a little too large for the hull speed, but I can cruise at top speed of the old yamaha, which was 31mph @6000 revs the Suzuki is doing that @4400 revs.

Top end on the Suzuki was 49mph, flat calm, tide and wind in my favour @6200 revs. No chine walking with those conditions, but more dramatic in a swell....

The other fact being that the fuel consumption is now slightly better with the Suzuki, as I'm cruising comfortably at much lower revs.

OK, is it legal, is it insurable, is it whatever, the difference in the boat is so much better.

Also point to think about, make sure the engine when operating the full tilt, has room to operate in the splashwell when fully up, so the leg/prop is not part submerged in water.