How fast should this boat go?

[Velislide]

Recently got my boat in the water, it's a 1984 18.5' Sylvan aluminum v-hull. Has a 1983? 90hp merc inline 6 on the back, 9.8 merc kicker motor, 20 gallons of fuel, two passengers (325lbs total), one battery. I've cleaned the carbs, seem to be all synced up. Motor runs great, though can't get my tach working correctly. Keep reading the signal wire on the tach hooks up to the rectifier on the motor? Not sure, but I'll attach a photo of where it was hooked up on the motor when I got it. Anyways, wide open Im hitting 35-37 on gps, throttle down to a cruise I'm getting 25 mph. Read that people with this motor and similar boats are getting over 50 mph. Not sure what pitch prop I'm running, will have to look And see if it says somewhere on it. First boat I've owned. Anything I should check? My cables are allowing carbs to fully open, checked that. Gray wire was running to tach. I reconnected it like that, sits on 3k rpm all the time.

 

[roscoe]

Speed sounds right to me. Have had several similar 18' Starcrafts, with I-6 Merc 90's and 115's.

 

[Texasmark]

If you are talking about me blabbing about what I get, my boat has a stepped hull like a fiberglass bass boat with a pad. The sides are short and it has no windshield per se so I have less wind drag. It only rides on an area about 18" wide which results in low hull drag. When hooked up the spray leading edge is at the corner of the transom indicating that there isn't much boat in the water. Second my 90 is 3 cyl and prop, not powerhead rated so I have 10-15 % more ponies than you with everything else on the engines in proper working order. I think 1989 or thereabouts is when the ratings changed. Third I don't have an alum prop. I have a high performance SS Ballistic. So sir it's apples and oranges.

On the tach, could be the tach or the rectifier. Chris 1956 is up on engines of that era and can help you better on where the drive signal originates. On my later model it comes from the rectifer/regulator module. What you might do is to loosen and retighten all connections on the rectifier module in your picture and crawl under the dash and do the same on the rear of the tach.

HTH,
Mark

 

[Watermann]

Sounds just about right for what you have upper 30's, maybe playing with the trim could get you a couple more but she looks like a fishing boat to me rather than a speed boat. I bet you spend the most time in the boat fishing with that bow mount. Besides most guys over blow and boast about what their boats/trucks/cars/snomobiles/ATVs will do for speed, power, gas mileage, hill climbing, wake jumping, wave busting, trailer pulling.... anyone can say anything on the internet. My favorite man boast is guys that say they get the same gas mileage pulling a 10,000 LB trailer as they do pulling nothing.

 

[H20Rat]

37 mph on that is actually quite good, consider yourself lucky! You could probably double the horsepower and still not get up to 50 mph, just not the right hull for speed.

 

[Velislide]

Was hoping to get at least 40 out of it. My prop is pretty chewed up, maybe I'll try and find a good SS prop. If my chewed up aluminum prop ends up being a 19 pitch, would changing to a 18 pitch SS net me a few mph maybe? I suppose it's hard to say without knowing my rpms @ wot. Been using the boat on Lake Superior.

 

[oldjeep]

A non chewed up prop would help. You haven't got enough horsepower for Aluminum vs stainless to matter.

 

[NYBo]

Was hoping to get at least 40 out of it. My prop is pretty chewed up, maybe I'll try and find a good SS prop. If my chewed up aluminum prop ends up being a 19 pitch, would changing to a 18 pitch SS net me a few mph maybe? I suppose it's hard to say without knowing my rpms @ wot. Been using the boat on Lake Superior.

Repairing your current prop would be more cost-effective. Get it fixed and see if you are satisfied with the performance. You can carry it as a spare if you decide to go with SS later. Always a good idea to carry a spare (along with a wrench and spare nut).

 

[H20Rat]

Was hoping to get at least 40 out of it. My prop is pretty chewed up, maybe I'll try and find a good SS prop. If my chewed up aluminum prop ends up being a 19 pitch, would changing to a 18 pitch SS net me a few mph maybe? I suppose it's hard to say without knowing my rpms @ wot. Been using the boat on Lake Superior.

A perfect prop, light load, and good water conditions and you MIGHT get 40 out of it. In any case, you probably don't want to run at WOT for long anyway, your fuel tank will go amazingly fast!

 

[sublauxation]

Your speed is about right. I've got the same boat with no windshield and an '06 90 on it. With just me and 12 gallons gas I get the low 40's GPS when trimmed up on flat water. Not sure if this will help but right now I've got a 15 pitch Stainless prop on back, I don't know which brand. I had a 17 pitch turning point aluminum that didn't get me enough RPMs.

Nice boat by the way! If you wouldn't mind adding a couple pics of the interior and the consoles I'd greatly appreciate it. I bought mine as an empty hull and haven't seen one in original form. I just got that same windshield and am hoping to redo things to add it this fall. Does yours have a livewell under the drivers seat? I pulled out that back splashwell and added a flat platform and it really opened up the boat.

 

[Texasmark]

Sounds just about right for what you have upper 30's, maybe playing with the trim could get you a couple more but she looks like a fishing boat to me rather than a speed boat. I bet you spend the most time in the boat fishing with that bow mount. Besides most guys over blow and boast about what their boats/trucks/cars/snomobiles/ATVs will do for speed, power, gas mileage, hill climbing, wake jumping, wave busting, trailer pulling.... anyone can say anything on the internet. My favorite man boast is guys that say they get the same gas mileage pulling a 10,000 LB trailer as they do pulling nothing.

Not rocket science. The wake in my Avatar says it all. Go to go-fast.com and pull up their prop slip calculator. Plug in 2.33 gear ratio, 24P prop, 5800 rpm and max possible (theoretical with no slip) 57 mph. Plug in some slip like 12 to 15% and you get 48 to 50 which is right where I am. I had to work at the setup and all to get there.

Mark

 

[pckeen]

Speed sounds about right for the setup you have.

 

[sublauxation]

Not rocket science. The wake in my Avatar says it all. Go to go-fast.com and pull up their prop slip calculator. Plug in 2.33 gear ratio, 24P prop, 5800 rpm and max possible (theoretical with no slip) 57 mph. Plug in some slip like 12 to 15% and you get 48 to 50 which is right where I am. I had to work at the setup and all to get there.

Mark

Well if we start with a 30P prop we could get him over 60! Only problem is it would be like one of those airplane movies where they're throwing all the extra weight overboard trying to get over the pass, the OP would have to pitch the windshield, both consoles, the deck, probably even cut half the boat off to turn any RPM's with a 24P prop

 

[Texasmark]

A non chewed up prop would help. You haven't got enough horsepower for Aluminum vs stainless to matter.

Not so!

 

[Texasmark]

Like Smoking crater said, that boat is made for fishing, not necessarily running all out. Too much hull drag, to much wind resistance, and currently, wrong prop.

Mark

 

[oldjeep]

Not so!

OK - explain how a stainless prop is going to help in an application where there is no prop flex? Unless that 90HP that likely puts out 50HP has some magic properties.

Unless you are saying that a chewed up prop is better, hard to tell

 

[Mel Taylor]

I maintain that the best way to gain both speed and gas mileage is to lie about it.

 

[Bob_VT]

If you visit evinrude, yamaha, or mercury web sites they have performance bulletins. Look up a 90 or 115 motor and look at the hulls they were tested on. You will find that your motor is in the right speed and aluminum fishing hulls are always slower.

 

[Texasmark]

OK - explain how a stainless prop is going to help in an application where there is no prop flex? Unless that 90HP that likely puts out 50HP has some magic properties.

Unless you are saying that a chewed up prop is better, hard to tell

Thin blades, cupping, high rake for bow lift (for boats that can benefit) and ports for good hole shot and high pitch for top end!

 

[bobdec]

Ever get your tachometer running ? what model is it and can you see what setting you are using on the tac ? The pic looks like gray wire is on the stator output, Mercs are 12 pole, is your tac set up for that ? Knowing WOT RPM would help in prop selection .