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13K views 67 replies 22 participants last post by  HookSet Harvey 
#1 · (Edited)
#2 ·
My boat is the same length and set up (console and decking). With two adults and gear, 10 mph is not bad in that size boat. That's right in line with what I was running when I had my 9.9. Without the weight of a passenger in front, you may be a bit bow high and not getting max speed from plowing too much water. I added a whale tail to my motor and it helped get up on plane when fishing by myself.
20 MPH with a 9.9 is most likely someone guessing or bragging, but seems highly unlikely in a boat like yours. My 25 HP Merc only goes 25-27 tops with just me and light gear.
You also might try changing the position a notch and check your speed both ways.
 
#5 · (Edited)
It wasn't heavy, 150 maybe. I know it was fun to use, but the rivets leaked and we had to fix the transom by stop drilling and increasing the wood size on the transom. The max hp was 15, so the power and torque did take it's toll on the structure of the boat.
(note: I used to put a heavy rock in the front as it would porpoise terribly at times). if your running about 5000 rpm and not bouncing in the front, that's probably all your going to get out of it.

.....Years ago, we launched it in my friends small pond in his back yard. He about had a heart attack when he came home and saw us fishing.
 
#7 ·
I don't know what your really going to gain..It's only a 10 hp motor. You'll be lucky to gain another mph or 2 with anything you do to it besides a new carb. It sounds like it's running about the speed of any 14..15 ft. Boat with a 10 hp unmodified engine that I've been on especially with 2 guys and gear in it. My 15 ft. Sylvan with 2 guys and a 10 hp. Honda will only reach 10...11 mph.
 
#8 ·
Be careful just changing the pitch of the prop....if you have no idea how many rpm the engine is running it could be disastrous....i agree with the previous post...its only a 10hp engine...i have a 06 9.9 4 stroke merc on a 14ft boat..(boat is 280 pounds empty)...and with 2 poeple im running about the same...just myself in the boat its maybe 4 or 5mph faster....
 
#9 ·
Agree with the other posters. A 9.9 on your boat can only do so much. It sounds to me like it's already at peak performance outside of a bigger carb. Mine had a 15 hp carb which helped a bit, but only a few mph faster. The only improvement I gained was in adding a whale tail and that was only noticeable when I was by myself. With my weight, plus 2 batteries, gas tank, and motor all in the rear, it was struggling to get up on plane with my 9.9. The tail helped, but still wasn't night and day speed wise. It was more of a visual advantage not having the bow riding quite so high. 10-14 or so mph seems right for your setup.
 
#11 ·
I've had a few 14's and powered them both with a 9.9 and a 20 and both motors ran like a top. One boat was a older Starcraft with a riveted hull bottom and read with a GPS the 9.9 pushed it at mid 13's at best at WOT 5800 rpm. empty with just me and a battery and tank. Hung the 9.9 on my 14 Lund which was a bit wider but a smoother bottom and still only ran 13's. Hung a punched out 20 on them and they each went mid 20's running 6000 rpm's. I'd say your right on the $ with your speed. 30 mph.. gheesh!
 
#12 ·
I've got a 15 foot flat bottom with an older evinrude 15 horse. It has wooden decks and I usually always have my yeti cooler, two batteries, Trolling motor, a couple anchors and a tank of gas. With just me in the boat, I get around 13 mph. If I have my kids or a buddy with me, I get 10-11 mph.
 
#17 ·
I never had a speedometer. my most reliable source was the sheriff at Seneca as he chased me down and gave me an inspection. He asked if the motor had been modified. I told him i was second owner, and yes it was modified, but I did not know what was done to it. He said I was doing about 30, and he suggested i have it checked by a reputable boat mechanic to make sure it was safe. It was too much motor for the boat, because it cracked the transom in two places so we stop drilled it and reinforced it. We also added a wider transom board with better hardware. We re-hammered several rivets to slow down the leaks. It was very fast, but it really took the waves hard.

A good lesson here. Finding leaks in an aluminum boat is a pain in the butt. I finally had the greatest idea. The neighbor and I parked it in the driveway and threw a garden hose in it. I laid under it, he was in it...and we reset the rivets with a body hammer and dolly.
 
#20 ·
The boat is stored offsite and I didn't have time this evening to check the items I mentioned above. However many of you have mentioned a popular modification - slapping the carb, or rather the jets, from 15 horse on a 9.9hp. I gave this mod a cursory investigation when I purchased the boat and the takeaway is it wasn't a good fit for this application. Such thinking stems from the specifications of the motor. It's a 93-94 model year, in other words post-1986. In that series is looks like the 6, 8, and 9.9 are all essentially the displacement (same stroke and bore) for 12.8 cubic inches of displacement. The 15hp, while having the same stroke, has a larger bore giving it a 16ci displacement. Most of the 9.9-to-15 "conversions" I've read about are because in those instances the 9.9 and 15 have the same displacement, with the limiting factor being the carb. Any guesses if putting 15 horse jets on a post-'86 9.9 has any real benefit?
Without knowing a lot about your specific question regarding model year, I can only add that the max difference you will achieve under prime conditions with a full 5 hp upgrade will be 4-6 mph greater. Keep in mind that a normal persons walking speed is 3 mph. It really comes down to a personal choice as to whether it's worth it to you.
 
#19 ·
I have a BT guide deep V 14 ft tiller with 9.9 Mercury 2 cycle. I believe there are a lot of factors in how fast my boat runs on a given day. Smooth water and no wind I saw 14 mph on the GPS. I did do some experimenting with the manual trim. It made plane on that day easily. Most days with light wind and waves 10 to 12 mph is doing good. Add two passengers and larger waves 7 to 8 mph tops.
I thought about getting a larger carb but decided getting to my spot a few minutes earlier was not that important.
There are a good number of modified 9.9 hp motors or re numbered motors on the water with speeds unlikely with a stock motor. I rode on a 9.9 16' flat bottom with power trim that went 23 mph. At least that's what the sticker said it was.
 
#27 ·
That motor has escaped from someone's sailboat!

Seriously, you have way too much lower unit in the water. As long as the prop & water pump pick-up stay submerged, any more depth is drag on your performance. Wouldn't surprise me if there's a little speed to be had in a shorter shaft.

How tall is your transom?
 
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