850 on a trihull

[MGuckin]

Hi I am back. Been gone for a while. Things have changed here. Forgive me if I post in the wrong area here but I am not sure if this should be here or on non - repair but I figured I would give this a shot first.<br />Just picked up a 74 Silverline 15' Tri Hull with a 75 850 hung on it. Runs fairly well but performance is lacking. Not really concerned about speed just effiency. If I want fast I use my other boat which was set up thru here and emails via some advise from a few of the long time members, (1976 18'Starcraft Cuddy with a 150 Evinrude, had it up over 55 mph and yes I was going a wee bit over the suggested max. RPM and not quite stock but I am babbling here sorry.)<br />Anyway back to the Trihull. GPS Speed = 27 MPH @ WOT @ 4600. I know RPM needs to be higher to really get into the power curve. I am tearing it down this week and will start over with the basics. Starts hard and floods easy. Comp is good though. It has the pototential.<br />I guess my real question is where to start with the plate. I run my Starcraft at about 2" above the bottom (slight vee hull). Tri Hull's are totally different to me. I think I should be about 4" above the bottom which is where it planes.Kinda hard to explain the way this hull is made. There is a pad about 4" wide running half the length of the hull from stern to bow at this point. Right now the motor is just about even with the lower most part of the vee. I am installing a jack plate.<br />This boat, with this motor, s/b capable of speeds of 40+ easily.<br />Thoughts or suggestions?<br />Thanks,<br />Michael

 

[Laddies]

Re: 850 on a trihull

Probably not, most tri hulls are dogs, some of the better designed ones were almost a modified V in back and did make good speed if you had enough power to keep the bow up. They just weren't designed for much preformance--Bob

 

[rsharer]

Re: 850 on a trihull

i,ve got a 1969 glaspar 15 ft tri-hull with a merc800 on it it.speed it not a problem,nor is coming out of the hole.my problem is turning.anymore than half throtle and i skip across the water.backing off and throtling into turn doesn,t help much.i think its just to much motor.<br /> my cavitation plate is just level with the bottom of hull.i also keep the tilt 1 hole from the transom.any more tilt and i cant keep the bow down.<br /> i dont have a working tach or speedometer yet as this is a project boat so i can,t give you my top speed yet but i went from a 35 hp to a 80hp.i would guess its twice as fast.with the 35,I could pull a wakeboarder ok.<br /><br /> the few trihulls I,ve seen have the plate set just about even with the vee of the hull.id, be afraid to go much higher.your water intake is just below that.have you adjusted the tilt any?

 

[MGuckin]

Re: 850 on a trihull

Trim almost all the way in. Started in the middle hole. Bow was hanging rather high. Brought it down by tucking the motor. Even trimmed like this it will pull a descent holeshot. Alot of the weight of this boat is 238 lb. motor and 18 gallons of fuel. Squats at the transom fairly deep when sitting still. Figured if I jacked the plate up to about waterline level of the hull, when on plane, RPM's should increase. There are ways around water pickup loss from jacking motors. Also the set back of the plate should help offset some of the water loss.

 

[JasonJ]

Re: 850 on a trihull

One thing with Silverlines is they are pretty heavy. Also, I don't know about yours, but my Silverline has a lot of V, about 22 degrees by my figures. Takes a bit of juice to get it out of the hole (on the plus side is it it very comfortable, does not slap or slam like most tri-hulls). Also, I can turn hard over at full throttle and it digs right in and holds, with a nice lean to it. No slipping or skipping. I have a 125 horse, and I am really happy if I hit 40 mph. 38 is about average, but then again, I am not rigged for speed, I am rigged for fishin'. Check my link if you want to see what can be done to these old beasts. <br /><br />Edit: I do have my motor trimmed well out, with a hydrofoil. I have spent a lot of time working on mounting height, trim, prop, and weight distribution. I want to get Smart Tabs and/or power trim, but to be honest, it works just fine like it is.

 

[MGuckin]

Re: 850 on a trihull

It does appear heavy and your are correct it is very comfortable. Not wet at all. The combination that I have planes very fast and has pulled a skier up as it was when I bought it. Redid the carbs and timing today. Starts much easier. Don't have T&T so I got to play with holes. Ran it 2nd hole from complete tuck before just to keep it from porpoising. Yup it has the power to yank it out.<br />Compression ranges from 151 - 160<br />Bringing some weight to play with. Want to trim out as far as I can and still get the bow down. If this works and the rpms come up a bit, now 4600 WOT, I will jack the motor and start again. <br />Once I get it to the highest rpm capable of this combination without loosing cooling and handling, I will decide on a prop if needed.<br />I did check out your project. Nice job.

 

[JasonJ]

Re: 850 on a trihull

Just for a comparison, I have my motor on the 4th hole out, cav plate 1 inch above keel, and a hydrofoil on. This combo works very well on my rig.<br /><br />Click this link if you want more info on Silverline boats.<br /><br /> http://www.silverlineboats.org/