Anyone with massively oversized outboard?

[Balthazar]

Hi all,

I just bought a Humber Ocean Pro 5.5m but the engine I already had is a Honda BF150. According to the rating on the Humber website the rib can take up to 115HP. The weight of Honda 115HP, 135HP and 150HP is exactly the same, 220kg, the only difference is the torque.

Looking at the transom it seems that it has the same thickness as the rest of the much bigger ribs, about 60mm thick, with 2 stainless braces added by some of the previous owner. The transom is sandwiched between 2 stainless plates 2mm thick and braced to the stringers under the deck. I of course have no idea how sound the stringers are.

Do anyone has experience with this massively oversized outboard and what was done to cope with the torque and pressure on the transom? I plan to run her at 20-25knts this summer till we have the next lockdown in September and probably reinforce the transom over the winter.

Any advise will be welcome.

[Bern Hanreck]

Hiya ! Run it past your insurance agent and consider the consequences !

[Nasher]

I'd imagine the 115HP rating was given before the general move to much heavier 4-stroke outboards, and I'd be more worried about it's weight than it's output.

The effect on the boats handling with all that weight on the back could be significant.

I massively overpowered a Flatacraft Force 4 years ago, it was quick in a straight line on a flat sea, but was far too tail heavy to handle well.

Nasher.

[beamishken]

Think I'd be whipping off the stainless plates for a looksee, usually only done as a workaround for a transom too short or more often a rotten transom I'd investigate before fitting any engine .
Years ago overpowering was fairly common but in todays litigious world I'd probably think twice unless you can get robust evidence that all will be good.

[Balthazar]

Thanks for your answers, the engine that was previously on this boat (and probably the max correct spec) is the Mariner 115HP wight 170kg vs the 220kg of the Honda BF150. That is just a 50kg difference, not a massive one in my opinion so I guess no exaggeratedly difficult handling or side walk.

But sure, I have to investigate the transom more in depth for signs of rot or delamination

[Unknown]

Your not just exceeding the max HP, but the max weight too. Their website lists 115hp or 200kg.

I know your 150 is only 20kg over spec and I would imagine that the transom is tested to much greater than 200kg but it’s set at that for a reason. Depends on your use really, as mentioned above, your insurance is something to pay consideration too but in theory, I don’t suppose there will be a huge issue with that engine on the boat.

[jameswallace]

My ribeye 585 i had a 130ho on. It was lethally quick. Way over hull design speed and would try and throw you out the boat. It was good fun.

[fastasfox]

Suck it and see.

[pagick]

I have a SIB with over rated motor. It’s 40 hp max rec, 50 hp max if hauling loads constantly. But I have a 60 ho on it. My wreck was as your outboard. 40-50-60hp are all the same motor with software to retard the 60 down.
I have no problem with mine but SIB’s are very different than RIB’s.
You don’t have to use all you got.

[willk]

EDIT: Sorry, I misread the thread title - my bad!

[Seng778]

I have a 50hp on my Avon supersport 345 that’s rated for 30hp. My steering is in the very front so I guess that helps with the weight distribution. I also put the battery in the front and installed a below deck fuel tank in the center. I normally cruise at a little over half throttle doing 20-25 knots. I’ve only had it wide open once and did 40 knots on an extremely flat day. I’ve had the motor on there for the past 4 years and the only problem I’ve noticed is that the rubber at the rear bottom was wearing away. I’m not sure if it’s because of the speed, the weight, the age of the boat or my 300 pound cousin riding in the back when we go spear fishing or everything combined. I glued on some wear patches and it seems to be fine now. I rarely use all the power but it’s nice knowing it’s there if I need it.

[beerbelly]

my 5.5 humber destroyer has a mariner 115 ct and mariner 3.5 aux hanging off the back about 200kg and at rest it sits quite low at the back prob no more than 150mm of freeboard .you also need to take into consideration the stainless plates sandwiching your transom will have quite a weight.mine runs well carrying this weight my console and side by side jockey seats are well forward and the console also carries 160 litres of fuel .all you can do is try it and see where your at .it does look like it may be a little top heavy

[Daibheid]

At this point I’m firmly convinced you can definitely over engine a RIB with power and or weight. The scary thing experience taught me is the rig can be great 99% of the time albeit with care as it will always chine walk toward the top end unless heavily loaded. But that 1% when you encounter it will terrify you. One of the worst moment I had was an over engined boat suddenly caught in a wind vs tide race with breaking waves about 2m high and barely a boats length apart. The heavy aft end was almost enough to flip backwards as I eased foward going almost vertical each wave. Veering either side would have rolled us instantly but the slightest excess of power hitting each wave and the engine would have rolled us backward. Less weight and less power would both have helped but carrying fuel in a built in console tank and spare fuel in the bow all looks good on level transit. But stand on the tail and all of that is close to going past the centre of gravity and you won’t think of it until you’re in that extreme situation. So weight is not just a number on transom strength, centre of gravity is a major factor that 1% of the time. I’ve had a few Humbers around your size, I wouldn’t dream of putting that Honda on it.

[Jon Tallis]

Definitely talk to your insurers first. I have a 5.8 destroyer rated for 150hp, I had to have the transom rebuilt a few years back and took the opportunity to significantly strengthen it. I was offered a 175hp motor. Insurers said "absolutely not" until I told them about the strengthened transom, at which point the conversation was "no problem at all if you can get it endorsed by a surveyor". However feedback from a number of surveyors was "not a hope pal, I'm not signing that off"...... So 150hp it is

[gaelforce]

I'd avoid a F5 head sea as there is an obvious danger of flipping. Also I'd store a serious anchor and chain up front where I'd also push the crew