115hp on a 15' trihull new transom? yes/ no?

[CaptainD101]

Well im just finishing up the detail work on this 15' Riviera trihull and gelcoating this weekend. all new transom wqith 2- 3/4 ply drilling into boat, glassed, sealed with 5200, and bondo smooth. So Im on the hunt for a motor locally on craigs and found a 115 evinrude mostly freshwater. The boat is rated for 70 hp. I wanted to overpower, if i can find the right motor, any suggestions for this boat. I was looking at a older 90hp but its the same motor as the 115. or should I stick with something like a 50

http://i41.photobucket.com/albums/e274/DDENIS1000/IMG_0143.jpg
http://i41.photobucket.com/albums/e274/DDENIS1000/IMG_0144.jpg
http://i41.photobucket.com/albums/e274/DDENIS1000/IMG_0145.jpg

motor
http://orlando.craigslist.org/boa/2840321373.html

 

[JB]

Re: 115hp on a 15' trihull new transom? yes/ no?

Stay with 70 or less, Capt. Overpowering and/or overweighting are dangerous. That Riviera 15 should perform very well with anything between 50 and 70HP.

Good luck.

 

[southkogs]

Re: 115hp on a 15' trihull new transom? yes/ no?

Beyond the danger factor (especially on a boat like that one), If I understand hulls right - generally overpowering a tri-hull offers little to no advantage over the rated max. That boat, with a 70 on it will be more efficient and should be able to pull two skiers with two in the boat.

The 3cyl Evinrude/Johnson 70 is a sweet motor, and would work nicely on that boat.

 

[Frank Acampora]

Re: 115hp on a 15' trihull new transom? yes/ no?

That boat is almost a flat bottom and with a 115 you should get at least 50 MPH top end. However: Why do you want to power it with that much?

It will NOT be a well handling hull and if you want to take your friends for rides and scare them--possibly kill them--that'll do it.

Before hanging a 115 on the back, you had better know what you are doing. You will need to reinforce the transom, rebuild the stringers and floor, put in crossmembers, and solidly attach the seats to the structure.

Believe me from first hand experience: I have a 14 footer with a 125. I spend one hour running it and eight hours re-gluing it. These smaller hulls are not designed to absorb the stresses the higher speeds place on them and need to be significantly reinforced to safely handle the speed.

I don't know how old you are, but young men tend to think emotionally and with their little head. If you want to overpower for the same reason a young man wants a crotch rocket, then don't do it. If you want to think logically about it, put in the man-hours, experiment and see what the hull is capable of, and don't drive it recklessly, then yeah--go for it.

I have a lot of fun with my highly modified and overpowered 14 footer BUT---I only use it when there is light or no other boat traffic.

Later on when the website allows it again, I will post a couple of photos.

 

[Texasmark]

Re: 115hp on a 15' trihull new transom? yes/ no?

Back in the days of the "fat 60" OMC, a friend built a boat and put that engine on it. First time out, in salt water, he hammered down for his first hole shot and ripped the whole transom out. Engine was submerged and only hanging on by the cables and the boat was sunk up to the bow....for awhile. Got towed to shore before it totally sank.

Frank has good advice and so does JB. If you are over powered and know it and have an accident, you could be legally in harms way. Think about it.

Mark

 

[CaptainD101]

Re: 115hp on a 15' trihull new transom? yes/ no?

Alright with all that said I am most likely going to stick closer to the range of 70hp. What my main curiosity is if a 70hp weighs about 250lbs and a 90-115hp weighs 295 lbs. The 45 lb difference on my new reinforced transom shouldn't see any difference from the weight. Now the force of a 40hp more during a hull shot, will this be that significant of a difference?

I don't plan to run down the river WOT like a maniac with people but I would like to keep up with the guys in my boating group. I want to have something that will actually be fast (45 to 45+) but enjoyable if just wanting to cruise and ski. will handling be horrible at just cruising speeds,I would understand bad handling at top end.

"I don't know how old you are, but young men tend to think emotionally and with their little head. If you want to overpower for the same reason a young man wants a crotch rocket, then don't do it. " lol you know you were the same when you were 24 y/o. I been fast on the road, Im ready to do it on the water. I might try it, I would like to see what happens but yet dont want to overwork the project since its almost done.

 

[robert graham]

Re: 115hp on a 15' trihull new transom? yes/ no?

Maybe find a good older model Yamaha 2 stroke 70HP....228lbs.....just what you need!

 

[Frank Acampora]

Re: 115hp on a 15' trihull new transom? yes/ no?

Yeah--when I was in my twenties, I was the worst. I equated power with a big ***** and large testicles. (As a matter of fact---I still do. I ski on 200 cm snow skis because their length equates to manhood length. LOL.) I left the driveway at 60 and tried to double the speed limit signs.

That said---NO- you are not going to rip out the transom on the first hole shot but eventually--probably sooner than later-- you will get stress cracks in the glass and yes, the transom will weaken. So, some aluminum plate strategically placed is necessary. The 45 pound weight difference should not bother the boat but it will sit lower in the water at rest. AND, at cruising speeds, oh, say about 30-35, she probably will handle nicely.

While you will be able to "Keep up with the other guys'" be aware that at the same speeds, a 15 footer will not behave the same in wakes as a larger boat. Going airborne at high speeds is a distinct possibility as is getting the dry side wet.

Because it is a flat bottom with no turn fin, in my 14 footer, I can not steer over 50. I need to slow down to turn it. Wakes over about one foot are a thrill at speed and no, I no longer want to test my mettle, so again I slow down. But, That feeling at 55 and 60 in a small boat on relatively calm water is sure pleasant. And, I pay dearly for it: With the 125, at 60 mph I burn about 15 gallons in about an hour and a half. Then I go home.

Understand: While this boat is fun, it is not my primary hull and only gets occasional use. I test modifications to increase speed and when I get some extra time I will try to get it to 65.

 

[eavega]

Re: 115hp on a 15' trihull new transom? yes/ no?

Man, I have a 15' YarCraft tri-hull with a 70 hanging on the back. I get that boat reliably up to about 32 MPH, and have gotten it as high as 38 MPH in perfect conditions (flat, cold water). There are a couple of things you should know about a 15' tri-hull
1. at speeds above about 25 MPH, it will jar the fillings out of your teeth on the slightest bit of chop or wake.
2. On a busy day at the lake, it is downright scary because the boat (at least mine) doesn't have a ton of freeboard, and crossing the wake of one of those great big cruisers will launch the boat and/or put a wave over the bow.
3. For entertainment purposes, the 70 is an awesome fit for a 15' fiberglass tri-hull. All the kids want to ride on the tube when I am pulling, as the boat carves so well it will pretty much throw any of the if I really crank over the helm. Its got enough power to pull a 200+ LB skier up quickly, even without an adjustable trim.
4. I can go all day or over 20 miles of WOT on 6 gallons of gas, with a reserve (I've never run the tank dry).

Rgds

 

[Texasmark]

Re: 115hp on a 15' trihull new transom? yes/ no?

Yeah--when I was in my twenties, I was the worst. I equated power with a big ***** and large testicles. (As a matter of fact---I still do. I ski on 200 cm snow skis because their length equates to manhood length. LOL.) I left the driveway at 60 and tried to double the speed limit signs.

That said---NO- you are not going to rip out the transom on the first hole shot but eventually--probably sooner than later-- you will get stress cracks in the glass and yes, the transom will weaken. So, some aluminum plate strategically placed is necessary. The 45 pound weight difference should not bother the boat but it will sit lower in the water at rest. AND, at cruising speeds, oh, say about 30-35, she probably will handle nicely.

While you will be able to "Keep up with the other guys'" be aware that at the same speeds, a 15 footer will not behave the same in wakes as a larger boat. Going airborne at high speeds is a distinct possibility as is getting the dry side wet.

Because it is a flat bottom with no turn fin, in my 14 footer, I can not steer over 50. I need to slow down to turn it. Wakes over about one foot are a thrill at speed and no, I no longer want to test my mettle, so again I slow down. But, That feeling at 55 and 60 in a small boat on relatively calm water is sure pleasant. And, I pay dearly for it: With the 125, at 60 mph I burn about 15 gallons in about an hour and a half. Then I go home.

Understand: While this boat is fun, it is not my primary hull and only gets occasional use. I test modifications to increase speed and when I get some extra time I will try to get it to 65.

Frank you are toooooo much. Big Grin. Surprised you snuck this by the site cops.

Mark

 

[Texasmark]

Re: 115hp on a 15' trihull new transom? yes/ no?

Man, I have a 15' YarCraft tri-hull with a 70 hanging on the back. I get that boat reliably up to about 32 MPH, and have gotten it as high as 38 MPH in perfect conditions (flat, cold water). There are a couple of things you should know about a 15' tri-hull
1. at speeds above about 25 MPH, it will jar the fillings out of your teeth on the slightest bit of chop or wake.
2. On a busy day at the lake, it is downright scary because the boat (at least mine) doesn't have a ton of freeboard, and crossing the wake of one of those great big cruisers will launch the boat and/or put a wave over the bow.
3. For entertainment purposes, the 70 is an awesome fit for a 15' fiberglass tri-hull. All the kids want to ride on the tube when I am pulling, as the boat carves so well it will pretty much throw any of the if I really crank over the helm. Its got enough power to pull a 200+ LB skier up quickly, even without an adjustable trim.
4. I can go all day or over 20 miles of WOT on 6 gallons of gas, with a reserve (I've never run the tank dry).

Rgds

Good info and I totally agree. Back in the days when they first came out ('70's) I was there. Was as Eric said. I finally went to an 18' Caravelle with a 125 Johnny which had a deep V with lifting strakes and some weight to it. Only then did things settle down, but you still had to watch out for water over the bow. Guy across the street had a 15' with a 60 hp Johnson looper 3 cyl and one day he came home from a local lake with his trailer only. At least he and his 2 buddies got back with their lives.

Oh the dead weight isn't what gets you it's the twisting moment on the top of the transom caused by the torque put out by the prop and the distance the prop is from the top of the transom what really does it.....course bounding down a rough road with a rough riding trailer and the engine flopping up and down can add both problems to the transom...weight and torque.

HTH,

Mark

Mark

 

[metriccrescentwrench]

Re: 115hp on a 15' trihull new transom? yes/ no?

It will NOT be a well handling hull and if you want to take your friends for rides and scare them--possibly kill them--that'll do it.

I'll second that. If you want to go fast, start out with a hull that's made for it. Tri-hulls have their purpose, but it's not speed.