What's the worst that could happen...

[carbon footprint]

Say I had a 20ft cabin cruiser, GRP, about 800kg total weight, with a stated maximum outboard rating of 120hp.

Now, say I took out the blown 130hp inboard, cut the transom to build an outboard well and bolted a 175hp Mercury motor to the stern...

I can build and strengthen the transom to handle the weight of the outboard motor (fibreglassing/metal work are no problem for me) and the all-up weight of the outboard is nothing like the combined weight of the original OMC inboard and sterndrive, so I don't think any of that will be a problem, even considering shifting the weight aft.

But...will 175hp make the boat unstable/uncontrollable? The hull is a Shetland 570 - apparently based on 1970's offshore racing hull design (so I've been told) - but, before I go for it, set out to sea and potentially wind up dead, any opinions would be greatly appreciated.

Thanks very much

 

[ezmobee]

Re: What's the worst that could happen...

The best way to do it would be with an outboard bracket. Here's an example (granted this boat is aluminum) http://forums.iboats.com/showthread.php?t=516113 There are pre-made brackets available. I don't think 175 sounds excessive for a 20' cabin cruiser.

 

[Silvertip]

Re: What's the worst that could happen...

Lots of things go into the determination of what maximum HP should be for a boat. In your case you are overpowering by roughly 35%. At anything less than full throttle there is likely very little chance the boat would be unsafe. At full throttle however, things might change quickly and with no indication of what is about to happen. I would certainly research the hull and see what power options were available for it or if modifications such as you propose were ever done by others.

 

[carbon footprint]

Re: What's the worst that could happen...

Thanks very much for your comments.

I've looked into transom brackets before - they seem to be well used in the US but are quite rare here in the UK. Suppliers tend to be from the US and marinas/chandlers I've spoken to locally have never even heard of them. I did consider having one built from scratch, but the cost was prohibitive.

Thanks again.

 

[ezmobee]

Re: What's the worst that could happen...

Sorry, I missed the "rated to 120" part. I only saw the removed a 130 part. I would think a 20' cabin cruiser would be rated WAY over 120HP. If it isn't though, I wouldn't exceed it.

 

[JB]

Re: What's the worst that could happen...

Sorry, I missed the "rated to 120" part. I only saw the removed a 130 part. I would think a 20' cabin cruiser would be rated WAY over 120HP. If it isn't though, I wouldn't exceed it.

Agree. Rating a hull for max power is not rocket science, but it is not algebra I either. Unless you are a degreed Marine Engineer please don't risk your life and the lives of others on SWAGs. (Simulated Wild Approximations and Guesses).

When there is a crash the world looks for someone to blame, and it has a rope.:redface:

 

[carbon footprint]

Re: What's the worst that could happen...

All fair points - thanks very much.

I've asked quite a lot of people about rating a hull for a particular engine power and the general opinion seems to be it's more to do with transom strength than anything else -I've never really come across a definitive answer...

Thanks

 

[ezmobee]

Re: What's the worst that could happen...

seems to be it's more to do with transom strength than anything else

Actually no. There are numerous factors. Check out this document http://files.dnr.state.mn.us/education_safety/safety/boatwater/backyardboatbuilders.pdf specifically the "safe powering" section.

 

[dwco5051]

Re: What's the worst that could happen...

Actually no. There are numerous factors. Check out this document http://files.dnr.state.mn.us/education_safety/safety/boatwater/backyardboatbuilders.pdf specifically the "safe powering" section.

Also you are changing the weight distribution by removing the engine from closer to midships and hanging it on the transom. This could make a difference in the handling. Not saying either way but it could decrease the safety factor.

 

[5150abf]

Re: What's the worst that could happen...

The baot was designed as an I/o, never intende to have weight hung from the transom as the drive on anI/o is fairly low on the transom and an outboard is at the top, totally different load so just changing from I/o to OB is enough of a risk, to overpower too seems kinda silly and very possibly dangerous.

As Silvertip said , when a transom goes it is normally at WOT and it happens very quickly and there is no recovery, the best you can hope for is that no one gets hurt.

SO in the end it is your boat and you can do what ever you would like with it but the opinions here seems to be pretty much the same, this is not a super good idea.

 

[bassman284]

Re: What's the worst that could happen...

The thread title reminded me of an old Calvin and Hobbes cartoon. Calvin has come up with a great plan involving his mother's car and says, "What could possibly go wrong?" Hobbes picks up his tail in his paw and says, "Every time you say that my tail gets all bushy."

 

[nwcove]

Re: What's the worst that could happen...

a 20ft cabin cruiser......weighing in at only 1764 lbs?? does that seem right? the i/o setup must weigh at least 25% of that??

 

[Willyclay]

Re: What's the worst that could happen...

My boat has a capacity plate from the manufacturer listing Maximum Horsepower as 150 although the USCG formula provided in the post above produces a result of 175. I read on the Shetland Owners Association forum that one member had a 90 on his Model 570 and he said it was scarey fast. Be safe and let us know what you decide!

 

[Sprky]

Re: What's the worst that could happen...

Most likely, the car you own will run well over 100mph. Do you drive it that way?????

You don't have to drive your boat WFO every where you go.

If you listened to everyone around here you would be driving a Prius and paddling a kyak!

 

[skyaggie]

Re: What's the worst that could happen...

Just throwing this out there but aside from structural integrity of the boat, are there legal ramifications for upgrading the horsepower of a vessel over its maximum?

Can you be cited for this?

 

[aerobat]

Re: What's the worst that could happen...

beyond transom strenght overpowering a boat may result that at WOT it comes to far out of the water and so looses its dynamic stability . or maybe nothing happens- nobody can tell you for sure.

but be careful- the power difference between a 130 hp inboard and a 175 hp outboard will be more than significant.

the next strange thing is the mass- my 21ft cabin cruiser weight with a 150hp evinrude ficht is 1450-1500 kg or roughly 3200 lbs .

 

[rob711]

Re: What's the worst that could happen...

Worse thing to happen...you die........die wondering how much that boat would have kicked
butt! Just do it, and consume great amount of alcohol during the build, oh and do it in a
poorly lit area, so it will look good. Once your done, take it out and just mash the throttle,
please video this for us. Ignore the safety conscious.....do it

 

[NSBCraig]

Re: What's the worst that could happen...

20' boat horsepower ratings no longer apply.

 

[Frank Acampora]

Re: What's the worst that could happen...



Well, Carbon---You will get a lot of opinions on this, some very conservative and some very wild. Now, Not knowing the laws there in the UK, I can not comment on them and it is up to you to investigate.

Here in the US, a hull over 20 feet long has different requirements than one under 20, so your horsepower requirements may be only recommendations.

Now, here is a 14 foot hull that was originally rated for 30 or 40 Horsepower. I now have 125 on it. HOWEVER: I did my homework and tested it with various engines of lower ranges before installing the big one. I also almost rebuilt the entire hull to accomodate the increased HP.

My 20 Foot cuddy cabin is "rated" for 170 and inboards here are not rated at all. You can have a 16 foot hull with a 500 HP monster engine installed.

SO: If you take a logical and cautious approach and do some judicious reinforcing of the transom, the 175 merc on your boat just may perform well. Of course, if you just decide to hang the engine on the boat and damn the torpedoes--full speed ahead, you may very well end up hurt or hurting someone else.