Converting to twin outboards from original single transom

[buckage]

This is my first post here, but I can see that there are some great responses to questions. I currently had a 24ft Offshore brand center console (Yes Offshore is the brand name manufactured by the OffshoreYachtINC.) Unfortunately they went ouf of business in 1994. It is a geat dry deep v hull and the transom is solid. It currently has an Evinrude 225 '91 model that weighs 443lbs. That replaced an '89 SUzuki 200hp that was about 460 lbs. the transom is not straight across...it drops 4 inches in the center where the current OB is mounted. I just purchased twin Johnson 150 Ocean runners, 1997 and am looking to mount the twins in place of the single 'rude. The extra 4 in or rise on either side allows the cav plates to sit about 3/4 inch above the bottom of the hull on each side.....looks like a perfect fit and that is also about where the single cav plate was aligned when in the center. DO I have to beef up the center opening when adding these twins? they weigh 370lbs each. the only spec rating for this boat on max HP I can find online is 635...so I am thinking they must have been rated for twins. MIC # is OSY89Jo8L989. What else besides toe in should I consider? I also have dual batteries now that are switched...will I have to separate them to run the twins? THese thing are a real bear to lift and move around but I hope to get them mounted this weekend. Appreciate all comments as well as if these are good motors....

 

[Philster]

Re: Converting to twin outboards from original single transom

Short of ripping up the floor, beefing up and rebuilding the transom and adding knees to better attach the transom to the boat's structure, I am going to vote 'NO' on this.

My reference? Some Hi-Po outboard boat builders who-over engineer their transoms for big HP and future bigger HP would never say you can add another O/B unless it was designed like that from the start.

If you are certain it was built for twins -- really certain -- scrap the above comments.


.

 

[buckage]

Re: Converting to twin outboards from original single transom

Well I sure hope I don't have to go there...completeld redid the stringers and floor last june with 5/8 marine ply. 5 layers of glass, fibercil'ed all the sides in. What do you mean by Knees? I was planning on atting 1/4in steel plate front and back of the transom and adding the same angled aluminum across the top of the transom. Is the additional280 Lbs that much of an issue or it the bigger issue the torque and thrust? Wouldn't think that would increase that much?

 

[Jeep Man]

Re: Converting to twin outboards from original single transom

"Knees" would be the L shaped brackets that attach to the transom and the bottom of the craft.

 

[samagy16]

Re: Converting to twin outboards from original single transom

This is my first post here, but I can see that there are some great responses to questions. I currently had a 24ft Offshore brand center console (Yes Offshore is the brand name manufactured by the OffshoreYachtINC.) Unfortunately they went ouf of business in 1994. It is a geat dry deep v hull and the transom is solid. It currently has an Evinrude 225 '91 model that weighs 443lbs. That replaced an '89 SUzuki 200hp that was about 460 lbs. the transom is not straight across...it drops 4 inches in the center where the current OB is mounted. I just purchased twin Johnson 150 Ocean runners, 1997 and am looking to mount the twins in place of the single 'rude. The extra 4 in or rise on either side allows the cav plates to sit about 3/4 inch above the bottom of the hull on each side.....looks like a perfect fit and that is also about where the single cav plate was aligned when in the center. DO I have to beef up the center opening when adding these twins? they weigh 370lbs each. the only spec rating for this boat on max HP I can find online is 635...so I am thinking they must have been rated for twins. MIC # is OSY89Jo8L989. What else besides toe in should I consider? I also have dual batteries now that are switched...will I have to separate them to run the twins? THese thing are a real bear to lift and move around but I hope to get them mounted this weekend. Appreciate all comments as well as if these are good motors....

I don't have much to add except that I highly doubt that the boat is actually rated for a Max of 635HP. That sounds VERY HIGH for a boat of that size. Hell, it's not even common place to find a 24' designed and rated for twins in the first place. I have a 2003 26 footer with dual Yammies rated for 450HP max, the newer model of my same boat 2004 and beyond is rated for 500Hp max which would be dual 250's. Now compare that to a 24 footer supposedly 635HP.

Be carefull with this, it doesn't seem right. Good luck!

 

[ceejkay]

Re: Converting to twin outboards from original single transom

take a look at nada for boats.. thats a great place to start, it will tell you all the options you could get your boat in. also do you have a plate that tells you max persons and hp??

 

[Philster]

Re: Converting to twin outboards from original single transom

My boat is rated for: Unlimited Horsepower. That's the builder's rating and official rating.

Checkmate goes to extremes to ensure you can hang the biggest engine AND put it on an extreme setback bracket, too, which means the transom is over-engineered. It (unlimted HP rating) is their calling card, so to speak.

I just can't speak for the OP's transom/boat without more knowledge. If it was engineered for twins, you have to know the max HP and determine what the builder knew at the time. Engines went from simple and light and get incredibly heavier over the years, so engines could outgrow boats and what builders knew at the time.

Now the trend is to trim down the pig 4-strokers, but there are many variables to consider. If there isn't much info to be had on the boat in question, proceed with extreme caution.

 

[buckage]

Re: Converting to twin outboards from original single transom

Ok did some research as you guys have suggested....Here is what I found. The boat does not have the USCG rating plate...It is very hard to find anything on the web about the boats except....

Company Name Offshore Yacht Inc
Current status of company
Company Status Out of Business
Date In Business 2/2/1983 0:00
Date Out of Business 5/12/1998 0:00
Boats Specialized in and Services Provided
Products Inboards- Cabin Cruisers

Manufacturer Identification Code OSY


NADA lists the boat at 2500 lbs with an 8.5 beam.

Here is the Iboat post on it..

1989 Offshore by Dynasty (FL) 24VCC
The 1989, 24VCC is a 25 foot outboard boat. The weight of the boat is 3220 lbs. which does not include passengers, aftermarket boating accessories, or fuel. The max rated horsepower of this boat, as listed by the manufacturer, (according to records we have) is 650 hp . If you do not have the manual for your engine then we highly recommend that you get one as repowering is costly and it takes you off of the water. When repowering, it is best to verify this information for the sake of safety and to ensure that your insurance company will cover you.


I don't think it is 650...My son in law says he remembers that prior to use completely rebuilding the stringers and floor, that he thought there was a plate showing max HP at 350 and 7 passengers. The transom is 2.5 inches thick and also has what appears to be another 2x4 brace for an additional 2 incnes at the top of the transom. THe two center stringers that run the length of the boat also are glassed to the transom perpendicular to it. The glass is heavy roven woven a is between 1/8 and 1/4 inch thick and solid all over with no rot or soft spots I can find. I plan to put an angled piece of 1/4 aluminum across the top of the transom all the way across (60 in), runinng down to the floor inside the transom and then triangle braced about 12 in out into the floor os the boat. If the motors can pull that off, they will have to take the floor and splashwell along with it. Is that over kill or just extreme peace of mind? Appreciate your thoughts and thank you to all that responded.

 

[ceejkay]

Re: Converting to twin outboards from original single transom

im no expert with the build, and with out a plate im really no help.. but what if you looked into doing a transom extender?

 

[buckage]

Re: Converting to twin outboards from original single transom

Thank you all for the suggestions.....I will be looking into the cost of fixes for this and post some pics of the project next week!

 

[NYBo]

Re: Converting to twin outboards from original single transom

NADA says your boat was made by Offshore/Dynasty Marine Group, has a dry weight of 3670# and a beam of 9'1". Offshore Yacht Bulders, Inc. was apparently a totally different company.

 

[buckage]

Re: Converting to twin outboards from original single transom

Measured beam is 8.5, just as it says for the 89 VCC model Hull ID of OSY is Offshore Yacht Inc. which was bought and also operated by Offshore Yacht Builders, inc. (Both companies had the same senior officers on their board, both started business in 1983 and went bankrupt in 1998. USCG site has the correct names. Dynasty also bout and sold several of the same boat hull's from OSY. Would love to see the NADA you are referencing and also any boats or pics of the Offshore brand....they made mostly high end yachts but did make powerboats for 4 years, Dynasty started in 1990 using some of their hulls ,but that is about all I have been able to find. No specs, pics etc. on anything I have seen nor other referenced to HP or # of Personell ratings...

 

[NYBo]

Re: Converting to twin outboards from original single transom

Here's a link to NADA that I found with your boat model:
Offshore 24 VCC

No such model listed under Offshore Yacht Builders. Not that NADA is infallible by any stretch of the imagination. Are the first three characters of your HIN OSY?

 

[buckage]

Re: Converting to twin outboards from original single transom

Yes...thanks for the link! My Hin is OSY24j06b989. When I tried to see NADA, they didnt even show the 89 so I appreciate the link.....the plot thickens. but the measured beam is exactly 8.5 at the widest point....I do think your weight is more accurate though. had it scaled with a full tank of gas (110 gal) the, triple 30 ft trailer and the motor and it weighed out at just under 6500 lbs.

ever seen one like in the pics I added above?

 

[oops!]

Re: Converting to twin outboards from original single transom

a few things.....

first, i like your boat.

the hp rating is not unlimited......in fact it is a coast guard rule that is basicly length x beam....
uscg rules here......http://forums.iboats.com/showthread.php?t=191283

due to the fact that your hull is soo deep.......and in gusseing that the deck is normal height..... you will have a lot of stringer attached to the transom....

if you really want to make sure abot the stregnth for the twins......
add two knees in the center of each mounting hook......the knees would be glassed to the deck.....(stringers is better) and go half way between the transom to and the deck....(the higher the better)

the glass layup would be csm.....1708.....csm....1708....

i would also re tab the transom on the sides.....
that layup would be csm......1708.....
12 inches on the transom.....12 on the sides.

done.........just make sure your prep is good

 

[buckage]

Re: Converting to twin outboards from original single transom

Thankis Opps for the great suggestions! I did do a quick read (7 Hours) on the boat build link. Wow what a nice job but I think way too muc h Corona went into it as well....anyway, spoke with my mechanic today. He said that because I had nearly 60 inchecs of open transom to mount on (motors only 12 inches wide), that moving them apart would easily counter the weight and thrush. He didn't see twisting as an issue either from the thrust. The counter rotating was a help. The two center stringers come up the transom about two feet from the floor and align almost dead center of where the new twins will mount. He felt that nothing needed be done but said that adding a motor to motor mounting plate across the inside of the transom would help balance the load of the two twins and provide something for the mounts to go through bedisdes just the transom itself. I didn't ask him how thick the plate needed to be. I am guessing a 5 foot plate on the back side (inside)of the transom will do and I am guessing 1/8" thick. He wasn't concerned either about adding the extra 300lbs of weight also because it would be spread out from the center where the 225 now sits which he says will also disperse the weight. There will be about a 14 inch space between the motors. I don't think I will be ripping up the floor, but I am leaning to also taking you suggestion of adding two knees on the inside of the stringers and up to the bottom side of the floor and glassing them in. I don't have the stranded 1708 I don't think but I do have about 300ft on a roll of some thin glass that looks like maybe twice the thickness and the same pattern of window screen. That is what was left from the deck remodel i did. While I don't like cutting any more holes in my new deck than necessary, the mounts for the twins will fall on either side of each of the two center stringers. When I glassed the floor in, I only left access to the inside of the stringers so I will have to cut out two more deck access holes in order to get to the outside bolts. Again, I hope to get started this weekend, if I can find a metal plate to start with to mount to the inside of the transom to bolt the motors through above deck. Would Aluminum be just as good a stainless for that? No need to add any more weight than needed.

 

[metriccrescentwrench]

Re: Converting to twin outboards from original single transom

... The two center stringers come up the transom about two feet from the floor and align almost dead center of where the new twins will mount....

As long as those stringers are solidly connected to the transom (I'm guessing they are), right there's your knees. It sounds like your transom is plenty strong, and it sounds like your mechanic knows what he's doing.

It'd be great if you could post some photos of your transom.

 

[R Socey]

Re: Converting to twin outboards from original single transom

My 1982 Wellcraft 248 fishernman is rated for twin 150s.