15 hp on a 25 ft o'day yes or to much
- Thread starter tom byrnes
- Start date
Definitely beef up the transom. My boat (wound up with two) O'day25, had a 15 four stroke bolted on it when I got it. The transom was deformed. The motor went with the previous owner and I put an 8 Johnson sailmaster on it. I went on ebay and bought two 12x12 1/4 plates. And a new motor mount. Pre drilled them at the house. I put one inside and one outside. It is solid as a rock now. Be warned, those nuts on the inside are a bear to get to. I am 6' 215 pounds and I got stuck in there. I had to crawl from the quarter berth under the cockpit and up through the front cockpit locker. I thought somebody was going to have to bring a chainsaw to get me out. My bout came from the factory with a water heater and it was tough to get under the floor. Here is a pic... Not a very good one but close enough. I think it would hold a 15 now.
A 9.9 would be sufficient - but if a 15 is what you've got it should be ok if you place a 1/4" aluminum plate inside and on the outside of your transom. You will most likely only max out hull speed (5.5 Knts) but you will only be at 75% throttle... I put a Gralick 10" mount on my 272 and it extends beyond the rudder. if your Mercury has throttle and shift control on the tiller, your good to go - if not you may want to invest in a cockpit control?
My Yamaha T9.9 is plenty for the 25mph winds and heavy chop I see on our bay. I have not yet needed more than 50% throttle to maintain hull speed. Weight is around 115lbs.
Beef up the transom as the others have said.
I would only be concerned with the shaft length and/or how much prop is in the water. Long shaft can work, Joe has proven that. XL shaft might be better as it would allow the OB head unit to be higher up. Short shaft would probably not work.
Beef up the transom as the others have said.
I would only be concerned with the shaft length and/or how much prop is in the water. Long shaft can work, Joe has proven that. XL shaft might be better as it would allow the OB head unit to be higher up. Short shaft would probably not work.
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After replacing the entire rotted transom backer (detailed in a long post a couple years ago),I epoxied in an additional 16"X16" square of 3/4" plywood on the inside of the transom at the mount.
Since you bought a 15hp, (hopefully long shaft) that's what you'll be using.
But if someone doesn't have a motor, and is considering a 15hp, I would suggest less HP, and using a high thrust prop. The change from a speed rated prop for planing jon boat to a high thrust on my displacement sailboat makes all the difference in the world. The motor has more bite at idle, and at WOT, I definitely get more RPM and get the boat past theoretical hull speed. Coming in to my slip, if I put it in reverse, it actually stops the boat moving forward.
5 hp Nissan 4 stroke on 18.5' boat. I don't feel the need for any more power.
Net gain, someone with a bigger boat than mine could use a 9.9 hp extra long shaft sail pro with high thrust prop, and get better mileage than a 15 hp with bigger carb. (usually the difference between a 9.9 and 15 or 20 is the carb size.)
But if someone doesn't have a motor, and is considering a 15hp, I would suggest less HP, and using a high thrust prop. The change from a speed rated prop for planing jon boat to a high thrust on my displacement sailboat makes all the difference in the world. The motor has more bite at idle, and at WOT, I definitely get more RPM and get the boat past theoretical hull speed. Coming in to my slip, if I put it in reverse, it actually stops the boat moving forward.
5 hp Nissan 4 stroke on 18.5' boat. I don't feel the need for any more power.
Net gain, someone with a bigger boat than mine could use a 9.9 hp extra long shaft sail pro with high thrust prop, and get better mileage than a 15 hp with bigger carb. (usually the difference between a 9.9 and 15 or 20 is the carb size.)
I checked a couple of O'day 25 documents I have that have the specs listed. The spec for an outboard is max 15hp and long shaft. I think I saw one document that list a max of 10hp but could not find that one this morning.
I agree with Joe and Brian on the high thrust prop for a sail boat.
So the answer to your Yes it is OK.
When I was looking for my O'day I looked at 23s and 25s. I found a 23 with a 25hp on it. I asked the owner why. He said it was much faster. I thanked him and moved on.
I agree with Joe and Brian on the high thrust prop for a sail boat.
So the answer to your Yes it is OK.
When I was looking for my O'day I looked at 23s and 25s. I found a 23 with a 25hp on it. I asked the owner why. He said it was much faster. I thanked him and moved on.