over the rated HP, but same weight

[Frenchie52]

I've been searching the forums, but was unable to find a definitive answer to my question. I'd like to put a new Merc 50 HP on my 16.5 ft Aluminum boat, which is rated for a 40 HP motor. The new Merc 50 and the new Merc 40 are exactly the same weight (204 lbs), so I was wondering how dangerous it would be to have that 50 on there. The boat's pretty heavy, so this isn't an issue of speed, but of functionability with 3 or more fishing buddies. What are the issues involved with over-powering this boat when weight is not one of them? Also, using JB's formula for calculating max HP (thanks JB!), this boat could be rated for a 100HP <br /><br />16 ft(L) x 6 ft(W)x2 - 90= 102<br /><br />But this seems really high. Any ideas?

 

[Dhadley]

Re: over the rated HP, but same weight

What did the insurance company say when you asked them?

 

[Frenchie52]

Re: over the rated HP, but same weight

They said as long as the weight of the two motors was the same, they "couldn't see a problem" with it. I'm not sure how much stock to put in that comment. I'm more concerned with the torque on the transom. I know it can handle the weight- will the torque/torsion on the transom be dangerous with 10 extra HP?

 

[mellowyellow]

Re: over the rated HP, but same weight

kinda shocking answer...normally if you exceed<br />the USCG max hp, they consider it negligence and<br />won't pay a claim, leaving you holding the bag.<br />1 possibility that may make it safe is that the<br />hull was built back when they didn't measure HP<br />at the prop. an old 50hp motor, if measured at<br />the prop today might come closer to 40 than 50hp.

 

[Terry H]

Re: over the rated HP, but same weight

mellowyellow...do you have firsthand experience that supports your claim that insurance companies don't pay in such a case as you mention. I've never heard such a thing except here on iboats. I've only known a few folks that ever had an insurance claim involving their boats,...hp was never an issue in such claims. Maybe you can share some info from the insurance industry that supports this? just a Thought

 

[mellowyellow]

Re: over the rated HP, but same weight

call your insurance company and pose the question.<br />I am sure they will explain even though common sense<br />on your part should not need them too.<br />I have personnaly driven a few overpowered boats<br />in my years, and the last experiment almost<br />flipped the tinny over...

 

[Frenchie52]

Re: over the rated HP, but same weight

mellow yellow- I'd be putting a new 50 HP on the boat, so the rating change doesn't help me. In fact, if the boat was built when motors weren't prop-rated, then the actual HP rating for modern motors would be more like 35 (since modern motors tend to put out closer to the actual HP on the cowling). <br /><br />Pre-prop rated max HP- 40<br />prop rated max HP- @ 35<br /><br />Make sense? I'm still wondering about the torque on the transom- there will typically be enough weight from the hull and the people in the boat to avoid a tipping problem, but the torque is what I'm concerned about. Any ideas?

 

[mellowyellow]

Re: over the rated HP, but same weight

frank, if u r an experienced boater I am sure<br />the extra ponies will not be a hazzard... <br />leagality is a totally diff. issue.<br />safe boating,<br />M.Y.

 

[Forktail]

Re: over the rated HP, but same weight

But this seems really high. Any ideas?

What type of boat do you have Frenchie, and what are it's exact measurements? Is it tiller or remote steering? Your calculation of 102 HP does seem high.<br /><br />In order to get 102 HP, your boat could not be a flat bottom or hard-chine boat (evidenced by a either having a "V" at the stern bottom, or not). And it must have both a 20" transom and remote steering.<br /><br />It appears the manufacturer of your boat used the basic CG Flat Bottom Hard Chine boat calculations, or GC HP charts.<br /><br />You can find out how the CG requires manufacturers to calculate their HP capacity here...<br /><br />http://squid.law.cornell.edu/cgi-bin/get-cfr.cgi?TITLE=33&PART=183&SECTION=53&TYPE=TEXT <br /><br />Another good reference, which includes HP charts and formulas, is Chapman's Piloting, Seamanship and Small Boat Handling book. <br /><br />According to them, it looks like 40 HP is about right.

 

[Frenchie52]

Re: over the rated HP, but same weight

The length from tip to transom is 16 ft. 4 inches. The transom is 20 inches high, and I'm not quite sure how wide the boat is at the transom...but the beam is 6 ft. 1 inch. Using the USCG formula, <br /><br />16.33 ft. (length)<br />x 5.0 ft (conservative estimate of width)<br />= 81.65 (or 82, rounded up)<br /><br />82x2-164-90=74 HP (rounded to nearest 5 HP, 75)<br /><br />in this formula, my max HP should be around 75 HP, with a conservative estimate of a 5 ft. wide transom. I think it would be closer to 5 and a half or 6 feet, with a six foot, one inch beam. I'll have to go and measure the transom to be sure. The boat is a console steer model. <br /><br />Somebody point out my math mistakes, if there is one...or, let me know if I've mis-understood the formula. Thanks!

 

[TELMANMN]

Re: over the rated HP, but same weight

Last year I decided to put a new motor on my old 78 sea nymph. I had an old 402 Mercury and I replaced it with a 50 Mercury classic. <br /> The mechanic who mounted the new motor decided to bulk up the transom with 1/4" aluminum even though the boat was originally rated for a 55 hp. <br /> I probobly could of got by with a new 40hp as the difference between the rating hp of the old to a new motor is quite a difference. I now can run the boat at 2/3 throttle with three people in the boat and still maintain plane with no problem. <br /> As to your issue I see three ways to go. The first would be to put on a new 40 and not worry. The second would be to put a fifty on and either find a different cowling from a 40(probobly difficult on a new motor) or third order new decals. <br /> In closing if worried about "CYA' go with a 40hp.

 

[walleyehed]

Re: over the rated HP, but same weight

Forktail, as I read your posted link, it appears they have made adjustments for consideration of newer prop-rated engines.<br />My 1979 boats (2 of the same size) are rated (BIA) for 150HP, but if I use the formula provided, my max equates to 130hp (126).<br />Considering I have already installed a bigger engine than that, would they go by the plate-rated 150 if an incident occured???

 

[mellowyellow]

Re: over the rated HP, but same weight

some reading for those interested:<br /> http://www.autoinsuranceamerica.com/WizWCCoverage.html <br /><br /> http://www.uscg.mil/d9/wwm/mso/msd/massena/ <br /><br />this last one tells exactly how coast guard comes<br />up with the max. hp ratings.

 

[Forktail]

Re: over the rated HP, but same weight

Somebody point out my math mistakes, if there is one...or, let me know if I've mis-understood the formula. Thanks!

Your math might be right Frenchie, but I'm not sure about the formula you're using. We still need to know what kind of boat you have, and its age. <br /><br />Most 16' aluminum boats don't have a "V" hull. Even though they might have a "V" bow, the aftermost length of the boat (where the boat rides under power) will be flat.<br /><br />If your boat has a flat bottom (or at least a hard chine) at the stearn, or certain design characteristics such as steering control location (maybe positioned on the side instead of the middle), and seating arrangement, etc., the HP may be reduced.<br /><br />Keep in mind that the CG formulas are for maximum HP based on calculation. They are mainly a weight consideration. But if the manufacturer feels the CG calculations end up exceeding what the boat can physically handle under real conditions (weight, transom stress, driver control, etc.), the manufacturer will reduce that HP to a safer level.<br /><br />Without knowing exactly what boat you have, it appears to me that the manufacturer is using the hard chine formula of (.5 X "factor") -15.<br /><br />BTW, make sure you're measuring your transom height from the bottom of the boat to where the outboard mounts, and not at the highest part. Many transoms are cut down. Also, make sure you're measuring the width at the stearn's aftmost.

 

[dakotashooter2]

Re: over the rated HP, but same weight

I can understand your questioning the validity of HP ratings. I have seen boat of identical size and design with the exception of one being console controlled and the other being tiller. The console models always have a higher HP rating. That would leave me to believe that weight distribution is also a determining factor.I these cases I doubt transom strength is an issue.

 

[Clams Canino]

Re: over the rated HP, but same weight

I don't think it's gonna be a safety issue. But it might be an insurance issue. I bet the stickers for the 40hp fit the 50 perfectly. <br /><br />-W

 

[Frenchie52]

Re: over the rated HP, but same weight

The boat has a v-shape at the back of the hull- all the way to the transom. The transom is cut-down, but measures 20 inches at the point the motor mounts. I talked with the tech guy at the marine dealers' today, and he confirmed what I found out at the Mercury website- the 50 horse and the 40 horse are the same motor, but carburated differently (I think that's what he said- someone with more knowledge can check me on that), and hence the weight is identical. In Canada where I live, the tech guy said that the max HP has nothing to do with legality; "just an educated guess", in his exact words. I asked the Manager and he told me the same thing. They have told me they'll install it on my boat, so that says something, too (I hope!).