15ft tri-hull and 50hp merc - is it enough engine?

[webrx]

Hi all,
I have a 1976 15' Steury tri-hull with a 1976 50hp mercury 500. With my buddy and I on it we were able to get on plane (400 lbs), though it minute to get there, but it planed. I added a deck to the front (3'x4' 3/4 inch plywood painted, sealed, and covered with exterior carpet - maybe 50-60 lbs). Took my wife and son out yesterday just for a lake cruise, maybe 500 lbs between the 3 of us. So basically I added 150 lbs, and the boat never got on plane. Engine runs smooth, idles, starts right up, ran at full throttle for an hour with no issues in terms of stumbling, bogging, missing - nothing, just would not get on plane. I adjusted the trim out one click (manually) to raise the bow a bit more, moved my weight to the back, tried everything but it just would not go on plane.

This is my first fiberglass boat, my 14' aluminum will plane my buddy and I with an 18hp merc, but the boat itself only weighs like 150 lbs.

So, before I start tearing into things - ripping up the deck to see if there is water logged foam, trying to increase motor rpm, link and sync, etc. I wanted to ask the question - should a 50hp on an 800 lb boat be enough motor to be able to plane out 5-600 lbs of people and gear or am I asking too much from this motor and I should be happy with what I got? I used to ski behind my buddies 16' hydrodyne v-hull with a 65 hp merc, and it never had an issue getting on plane, so that is my reference.

I read about weighing the boat, but I have no idea what the old trailer - solid steel, walking decks, etc weighs i am guessing a bit.



Thanks

Dave


Second question, for anyone that has one of these motors, I have found a few things the PO had tinkered with, one being electrical wiring, which I appear to have figured out - all but the tach is working now - tach works fine between 0 and 1500 but wont go any higher (engine RPM does, just the tach does not). I have attached a pic of the port side linkages at WOT (engine not running) I had to fix a couple things here (nut missing, idle speed screw not touching with throttle in neutral - idle screw looks 'different" to me - should it be longer - it barely makes contact when fully screwed in?). If you have one of these motors can you take a pic and send to me or look at my pic and see if anything looks out of sorts please?



Thanks again

d

 

[Maxz695]

Re: 15ft tri-hull and 50hp merc - is it enough engine?

What size pitch is the prop. If it,s 13 or more it may be too much and a smaller pitch would be recommended. You may lose top end speed a bit but the engine won,t have to work as hard and be stressed to the point of breakage

 

[ajgraz]

Re: 15ft tri-hull and 50hp merc - is it enough engine?

V-hull will holeshot and plane much easier than a tri-hull, so your reference is a bit skewed. Max's advice on prop pitch is good; I'd add, consider going to 4-blade for more holeshot (also at cost of top speed). You'd probably really like the 11-pitch 4-blade Comprop on that setup.

I have a mid-70's 70hp Merc on my 15' tri-hull, and it just adequate. Could not imagine using less hp than that. Boat is rated to 100hp, I sometimes wish I had a 90hp!

 

[Chinewalker]

Re: 15ft tri-hull and 50hp merc - is it enough engine?

If the boat isn't waterlogged, I see no reason why a 50 wouldn't be plenty for it. I had a 15' trihull (can't remember what make it was - it was blue and we called it the Booberry) and it came with a 50 Merc on it. Performed quite nicely. Used it as a workboat for a couple summers...

 

[Maxz695]

Re: 15ft tri-hull and 50hp merc - is it enough engine?

Believe it or not my procraft 1750 is rated for a 150 LOL Hold on to your hats!!!

 

[webrx]

Re: 15ft tri-hull and 50hp merc - is it enough engine?

boat is rated for a 90 hp, so, 50 is about half of what it is supposed to be able to handle. I bought new plugs yesterday, I pulled number one it and was pretty carbon fouled, maybe this will help. boat idles well at around 650, seems to purr at full throttle, no missing, no coughing, etc. just seems like it ought to do better getting on plane with two adults and a 11 year old in it. It is a 13p prop, but I dont know what RPM it is hitting until I can fix or replace the tach and dont want to overrev the engine. I did use a mirror and it appears the carb plates are wide open at WOT but I will need to pull the front plate on the motor to be sure. I guess I will pull up the carpet and see if there is an inspection plate so I can check for waterlogging, if not, then I will see if I can find someone local to walk me through a link and sync and go from there.

Thanks

d

 

[mr 88]

Re: 15ft tri-hull and 50hp merc - is it enough engine?

You are a underpowered [ by the formula method ] so with some possible/probable waterlogged flotation you should have a hard time getting it up on a plane with a average payload.Have you tried taking it out by yourself and all extra gear removed ? If not start there and see what happens. How is the compression ??

 

[webrx]

Re: 15ft tri-hull and 50hp merc - is it enough engine?

Thanks for all the responses, I was thinking I was a minimum hp for this boat being rated for 90 hp and weighing at 1k with the motor and having a wide hull. It is my first tri-hull and it is nice and stable when fishing, but, I know it is not a speed boat, but was hoping to be able to pull the kid on a towable occassionally. it will go 25 or 30 I guess (no signal so couldn't gps it) and that should be adequate for a towable.

Compression is good, 105 to 110, I have gotten it on plane with two people in it, before I added the front deck ~50lbs engine pinned on the second hole away from the transom. I am thinking the only real difference between then and now was about 160 lbs - boy sitting up front, but even when I moved to the back it still did not plane. Maybe I need to add trim tabs or a whaletail as I don't see forking out the money for a 75 to 90 hp right now. The boat will mostly be for fishing, hence the fishing platform up front, but being able to take the family out for a spin is also a desire - it worked ok, I just wanted it to get on plane to give the kid a little more speed adrenelin. I just looked at the deck, and it seems solid, no soft spots, but, that does not mean there is not something lurking below I know. I will probably live with it for the rest of this season and come spring will pull up the deck to see what is underneath. the boat is a 76 so it would not surprise me if sometime in the past the foam got wet but I get no water from the drain, even after letting it sit bow elevated for a day, and I would think if there was water intrusion or wet foam I would get something out of there.

the boat will plane with 2, and so I am fairly confident it will plane with 1, but I will give it a shot. I may also find a scale locally and see if I can take it off the jeep and weigh it without the weight of the car so I can get some kind of accurate feel for what it weighs. The boat is supposed to be like 775 and I guess that steel trailer is at least 3 or 4 hundred, motor is 200. So if I am in the ballpark of 1300 to 1400 I would think I don't have a major issue with the flotation foam. Does this sound right?

d

 

[Chinewalker]

Re: 15ft tri-hull and 50hp merc - is it enough engine?

Certainly won't hurt to go down in pitch a couple inches. Might see a marked improvement in acceleration with an 11P prop.

 

[webrx]

Re: 15ft tri-hull and 50hp merc - is it enough engine?

While I know there is a fine line between being able to plane or not, dropping to an 11 should give me more speed, but less hole shot correct? more speed means being able to get on plane easier? or am I missing the proverbial boat?

I am not pulling skiers so hole shot is not that critical, and I typically get the boat moving, then go to half throttle, then to full, in stages, no sense in beating up an older motor if your not in a hurry right?

 

[Maxz695]

Re: 15ft tri-hull and 50hp merc - is it enough engine?

Just the opposite the 11 pitch will give you more push with weight in the boat and less top end as the prop ratio to RPM is going to be moving more water faster in the hole shot to get you up there. There for top RPM will be pushing less water when it gets to MAX due to less pitch

 

[webrx]

Re: 15ft tri-hull and 50hp merc - is it enough engine?

OK, so please bear with me here, if I am having an issue getting on plane do I want a larger pitch or a smaller pitch - doesn't getting on plane require higher top speed vice faster hole shot?

I did some research, and it appears a smaller pitch will improve hole shot and max rpm will increast (400 rpm for every 2 inch decrease), and with higher acceleration it should be easier to get on plane - I think that is what I read

So, if I were to go with an 11 pitch, any particular brand? Good and bad with aluminum vs steel? Recommendations? Thanks

Also, I pulled the carpet up in the boat, deck is sound, no soft spots, I got up front where the front drain passage to the bilge is (sorry, dont know what is should be called) and with a flashlight I looked down that tunnel to the bilge pocket in the back. no seepage or cracks that I could see. So, short of drilling a hole in the deck, I think the next step is to weigh the boat to see if there is water weight in it.

As stated before, The boat is supposed to be like 800lbs and I guess that steel trailer is probably 300, motor is 200. So if I am in the ballpark of 1300 to 1400 I would think I don't have a major issue with the flotation foam. Does this sound right? How much weight would water impregnated foam add to a 15 foot boat? At what point do I say, yep I have a weight problem? I know a lot of this is guess work cuz I don't know the weight of the trailer, but I also don't have a way to take the boat off the trailer and weigh the trailer alone unless I am going to launch the boat, drive an hour to the scale, and then go back and get the boat.

Thanks for all recieved and any future advice

d

 

[Chinewalker]

Re: 15ft tri-hull and 50hp merc - is it enough engine?

Going down in pitch is akin to shifting down in a car. You would shift from 4th gear to 3rd when going up a hill - akin to getting up out of the hole in your boat. Although it is true that you will gain better hole shot, it is not always true that you will lose top speed when going down in pitch. Depending on set up and how far off your pitch is presently, a higher pitch prop may be lugging the motor enough that it is not reaching it's proper top RPM, in which case it will not be running in its power band. No promises, but it is possible you may see a top end improvement as well...

As for excess water weight, I took about 200 lbs. of waterlogged foam out of my 15-foot Starcraft during its rebuild. The foam was relegated to the space between the tunnel and the chine on mine...

 

[webrx]

Re: 15ft tri-hull and 50hp merc - is it enough engine?

Thanks again,

OK, an 11 pitch is on the shopping list along with a spare tire for the trailer, tach (replace/repair), and seats.

Doing more research, I am pretty sure I am at the edge of perform/dont perform with the engine size for the weight of the boat and as I said, with 2 it planes, with 3 it wont, I will play with it some more, but, the normal load for this boat will be one or two people, and I will use it primarily for fishing.

Thanks again.

Dave

 

[Maxz695]

Re: 15ft tri-hull and 50hp merc - is it enough engine?

Planing with 2 and not with three indicates going down in pitch is the right thing to do. With the tach and a full load it should range between 4800 and 5500 RPM AT WOT. There is something eles you will want to check for and maybe able to adjust. I have a jack plate on my boat. I,m not sure if your bolted to the transome or not where the LU meets the midsection should be in line or as close to it as you can get to the bottom of the rear of the boat. Too much and it will cause drag and too little will not let the prop grab sufficient water and possibly cavitate

 

[buddhapi]

Re: 15ft tri-hull and 50hp merc - is it enough engine?

IMO that 50 hp is going to be underpowered for that 15' tri hull. 65hp is about right for a 15' V bottom narrow, but a wide trihull like I have, you're best off with 90hp if you want to pull a skier or tuber. I'm working on a beautiful 1972 115 Tower of Power, that would be sweet on the boat you're describing. fiberglass boats are heavy.

 

[webrx]

Re: 15ft tri-hull and 50hp merc - is it enough engine?

Maxz, motor is bolted to transom, and I am pretty sure is original to boat from when it was purchased just based on what I have seen tinkering and cleaning everything up. Cav plate is right at or maybe a 1/2 below bottom of boat so not likely to adjust it at this point but I do understand the height alignment.

Budd, I think your spot on here, I believe the boat was purchased with this motor and whoever bought it was not "speed driven". I would guess that with everything working perfectly this boat is adequate for two and if I want more (skiing) then a new prop and/or motor would be the ticket.

I will most likely go to an 11 pitch to get a little more out of here, but not likely to swap engine. I need to get the tach working or a new one installed before making any prop changes, as I can't afford to burn this one up, and I am thinking I might take it to a shop to have the link and sync done, I may try this myself, but if I do may have to ask questions of someone who is familiar with this motor. I am pretty mechanically inclined, just never done this before and don't want to mess it up, I have rebuilt and repaired a lot of smaller motors (18hp to 2 hp), this is just my first big boat motor (though I have done cars and trucks) and the linkaes are far more complicated than the smaller ones.

Right now the boat will allow me to do what I want to do 99% of the time - that's get to a fishing spot and provide a stable and roomy fly fishing platform. the other 1% right now is to pull the kid on a tube and take the wife and kid on a lake ride - though I would be happy to do this more often. The prop and link and sync (gut feel tells me someone has messed with these adjustments based on what I have found under the cowl) will probably make the boat adequate for this and since it is not going to be the boats primary function, will have to be what it is until money gets looser (wife may have a full-time teaching job soon - cross your fingers for me please).

This is my first (first one I have owned and operated) fiberglass boat and yep, to me it seems pretty heavy and solid compared to the Tinny's I have owned. The 16' I learned to ski behind belonged to my buddy, had 15 more horses, and was a v bottom, it was relatively new at the time, did the job, and could pull two experienced skiers if they could get up quickly. But I weighed half as much then as I do now and so did my buddy

The primary reason for this post, and what I believe I have gotten out of it was to figure out if the 50hp was "on the edge" of adequate - I think it is, I think the Steury was built very solid, and is heavy (boat alone is 775lbs per the specs) compared to my 15' crestliner aluminum which is around 175 or so, not much more. the aluminum will get on plane with a 15hp, maybe a 10 but I have never gotten it to plane with anything smaller than a 15 (tried 7.5 and that did not get it there), even doing the math with this, ignoring the v bottom vs trihull 4x the rounded weight of the tinny - is 800, 4x 15hp is 60hp so the weight of the tri-hull is 800 and 50 is going to be barely adequate - I am gonna have to live with that for now, though I may be able to swing a new prop to make it "better".

Thanks for all the input everyone, I really do appreciate all the help and advice.

Tach first, and I will check back in once I have that resolved and I know what RPM I am turning.

Dave

 

[unior]

My Merc 500 will get my 16 foot glassmaster trihull on plane with 700 lbs of payload.

 

[Chris1956]

Gee, a 50HP Merc is a fine engine, and should plane that boat with 4 people, fairly easily. We used to ski on a 16 foot Deep Vee hull, with a Merc 500.

I would do a compression and spark test to make sure she is working on all cylinders.

 

[jbcurt00]

Closed

Webrx asked back in 2012, nearly 7yrs ago. Doubt he gets or responds to your posts, hence the 90day rule.

After 90days w no new posts by the OP (Webrx in ths case) treat topics as closed. Use it as a resource only and should not to be posted to.