Should I be going faster?

[cmikee]

I have a 1970 Mercury 115HP model 1150 on a 17FT Glasstron Fish and Ski Boat. I have a concern about the speed of the boat. The top speed currently is 34 MPH@about 5000RPM. The boat is fairly Heavy and made of Fiberglass. I have a 2 blade Bronze propeller, Im not sure about the pitch I think the diameter is about 13 inches. Thanks for all the help.

[Liqui-Fly]

Yes. A good solid 50 seems possible. Figure out what your prop pitch is first.

David

[cmikee]

How do I determine the prop pitch? I haven't looked at the propeller for numbers that may tell me the pitch yet. If there are no numbers then is it a calculation? Im still learning so please bear with me. Thanks

[action17]

also check for water the boat could also be water logged:eek: check transom and your stringers!!!;)

[cmikee]

I have a new transom made in 1999 and the stringers (which i believe are the long runners that run from the back of the boat to the front ) seem to be ok. Im not sure what to check for there. As for water logged im not sure what you mean There is no water in the hull. Thanks:)

[Liqui-Fly]

About your setup. A calculator isn't going to do much for us because we don't know a number of things. Like if your tach is off. How far is your propshaft below the boat? What other props have you run if any? ect ect.

David

[Laker]

Is the hull a 1970 also? what type? A Try hull, V hull?
How fresh is the motor? depending if the hull is more performance oriented... If not I would say your not doing too bad. Does it have troubles planing off or carrying a load?

[action17]

thats fine i think your ok there just trying to narrow the problem down!!!!

[cmikee]

The hull is a 1970 and it is a Try hull. The motor was last rebuilt in 1988 (probably needs another rebuild but runs pretty good). I can get out of the hole very fast and get up to 30 MPH fast. But after that it's about toped out. It planes out at little more than 20MPH. When more people are in the boat or a big load I really can't tell it.

[lorax]

For what it's worth..
I have a '73 Glastron V173 w/Merc 115 and a 12 3/4 X 21 prop.
The best it's done (measured with the boats speedo) is about 42mph. That's with two adults, no wind and water like glass.

[Laker]

Sounds like your doing about right.
You could take advantage of that motors torque with a better propeller... Does it have trim and tilt?

[cmikee]

Yes I do have Trim and Tilt:)

[tombrown]

Mike, the aqua-lift II hulls tend to be a little slower than the aqua-lift I hulls but they aren't far off. Unfortunately, there aren't many aqua-lift II models in the Glastron performance table. There has been some anecdotal information go through the CGOA message board that might help.

http://www.classicglastron.com/

I would think that your boat should be good for 38-40 with that engine. Do you have 2:1 gearing? If so, consider getting hold of a Mercury 17p 3 blade to try. It may be a wee bit too much but should be pretty close. If you could borrow both a 15p and 17p, that would be great.

Those 2 blade props tend not to be the best performers and some have mentioned that they vibrate more than the newer style props. That reason alone might make you want to get something newer.

Please consider sending me the particulars of your setup and performance so I can include it in the Glastron performance table. This information can be extremely valuable to others in the same situation.


Thanks,

Tom Brown
Glastron performance table maintainer.
http://www.carlsonspeed.com/Data/Performance/PerformanceTables.html
[email protected]

[TD]

THAT boat will never see a real 50 mph with that motor. Maybe 40 tops.
Gday
TD

[Raceman]

I'd ditch the 2 blade prop also for a couple of reasons. Number one, they don't do real well on a heavy boat efficiency wise and also they're hard on parts due to vibration in comparison with a 3 blade. The part number is probably on the side and likely has the pitch stamped along with it, just as 2 digits.

As far as the 70 model 1150, my first boat, a 73 18' Super Sidewinder had the same engine as a hand me down from my dad who wanted an excuse to buy a new 1500 for his Wellcraft. I almost immediately converted it to a 1350 and don't remember that much about the powerband, but even in 1150 form I think it'd like a few more RPM's than 5000, maybe 5600 or so. Merc's recommended RPM range on the 70 & 71 1150's and 1350's was 4800 - 5300, but it's better to let em turn a little than load em, especially with today's gas. Those engines had the higher dome pistons than the later stuff (late 70's foward) and gas/timing can become an issue. I'd be real hesitant to set the timing by the decal on the front bracket which may be as much as 27 to 30 degrees. The 70 engines (identical to the 71's) represented Merc's change from crossflow to the direct charge design. There was no difference between the 1150 and 1350 in these two years except the reed stops and the plastic diffuser in the middle of the carb throats. The diffuser part is easily handled with a set from any later inline carb, but the reed stops require powerhead disassembly. It's a change I'd definately make if you ever have it scattered again, as it really does seem to make a true 20 hp difference.

If your engine still has the original gearcase it's got a 1.78:1 gear ratio. If it was properly rebuilt and doesn't have tons of hours I wouldn't go in it just from a time standpoint. It's not unusual for these old engines to run up tons of hours if properly maintained and still have good compression and power. I like running a little extra oil in mine too by the way.

You'd probably expect to pick up a couple or three MPH with the upgrade to 1350 specs and the right stainless three blade might give you that much more again.

[Liqui-Fly]

I didn't think we were talking about the trihull.

David

[Karl817]

CMikee ~ by no means am I an expert on hydronamics or boating (other than cooler-management :D ) but what little I do know is this.
Engine tilt - you want to trim the engine all the way in for wide-open take-off and then bump the trim out until the boat starts to porpoise up and down then bump the trim down until it stops. If it's a manual type trim you want to move that little bar out a notch until you get the porpoise-action, then move it in 1. This is an easy way to get your boat up and outta the water. You might have the trim all the way down and that's making the boat plow thru the water.
Prop - Sounds like you have what I refer to as a power prop, made for takeoff but not much good for high speed cruising.
I think a 115 should push your tri-hull at least 40 if not close to 45. I have never seen a moter bigger than a 50 with a 2-blade prop.
My advice - play with the trim first, then find a different prop to borrow for a day or two and see what it does. If I'm not mistaken you want that 115 turning 5000 - 5500 WOT, and your prop has a lot to do with that (assuming it's tuned and good compression etc...)
But as I said up above, cooler-management is my speciality.:p

[Raceman]

Generally speaking, in the good old days when everybody ran multiple props on underpowered boats, one for speed and one for power, or pullin' as we called it, the power prop was a 3 blade that would let the engine turn up some R's. If he's only turning it 5000 it ain't no power prop. We can be more help with a starting point of the pitch that's on it now.

[Techno]

At 10% slip and 5000 rpms
1.78:1 gears its 36mph for a 15" prop
for 2:1 gears 34mph a 16" prop comes out.

This assumes your tach is right and your speed is right. Sure seems a low pitch.

[Raceman]

I agree with your figurin' Techno, but there are several variables here. First of all with that old 2 blade it may be slippin' more than 10% and a lot of blades back then weren't even cupped, so if it's a 2 blade aluminum without cup almost anything could be goin' on with it. Even if it's an old bronze wheel, it could be either way, and there ain't no tellin what diameter it is either. Those old speedo's from that period used to be real optimistic too, especially the Airguide brand. We used to call em happiness meters.

[Techno]

That was just to show a baseline of what the numbers say it should be.
Assuming the gauges are even close then the pitch is roughly in this zone.
I wasn't saying this is the pitch but this should be the pitch and if it isn't then something is in error.

[Raceman]

Hope you didn't think I meant all that as a contradiction to what you posted Techno. As usual, your numbers were dead right. I was just trying to point out some possible variations in those old props. I figured you might not've seen some of that old stuff from way back then.

[Mark75H]

I ride with lorax on this.

We sold those Glastrons back then, 42-45 is about all it is going to run with a 115.

Motor height has not been addressed on this boat and is critical. If the cav plate is even with the bottom of the boat it may not get much over his current speed. How much higher he can go depends on the exact prop.

As far as increasing motor power or expensive props....he'd just plain be better off trading that pretty darn good ski boat for something else if there is a serious need for speed.

[Techno]

I bought the Aeromarine prop calculator. Thought I was buying something else. Oh well my fault for not paying attention. I'm glad I got it now because you can change any of the variables without re-filling out boxes.
The RPMs are a slider so you can leave everything filled and slider up to find top speed. The problem is its too easy to keep tabbing bigger props and reaching higher numbers

It does show that this is really just playing with numbers. You put in the stamped on the prop 26". Is it really 26"? then there is cup changing it. So even with exact rpm and speed we never have the actual pitch and adjust the slip number to make it right.


For the prop though he should be able to get a fairly cheap prop for it. This pitch is something not too many people are using. I have a spinnelli modified round ear 23"? How many here can use this prop? Some boat/engine shops have an exchange thing, I've seen pretty cheap props there.

[cmikee]

WOW!! Thanks for all the input. I will take all this information and research it and see if I can get a little more speed. I like cruising around 30 MPH and it seemed to push it to do that so I fiqured if it could go faster I would burn less gas at 30MPH. I do have the airguide guages (from 1970 probably). But I have tested the RPM guage and it's correct. Thank you very much for the info. I will post and let you know what happens.

[dan agnew]

where in s.c. are you located? we are located on lake murray in newberry county. if you are anywhere near here i would be glad to allow you to try any of my props on your boat i kinda side with raceman you need to free up the engine turn more rpms and if you got a two blade i would bet it is not cupped so you are just thrashing the water and not going anywhere.
next weekend is the raindate for our outlaw perf boat rally. we meet at hollands landing on lake murray if you can come you are welcome to. we ride to the spot where we run radar runs[not this time thou] push the boats up in the lily pads and wade out in the lake lie to each other pop a few tops and at last rally we were allowed by dnr to pair up and run whoever you wished. do not feel like you would not be welcomed we have had river boats with 40 hp tiller handles. maybe we can help maybe not but you never know.
any ???s call my shop 803-364-9351 leave message if you get machine

Danny

[cmikee]

I tried to tilt the motor up a little and tried the top speed and got 38 MPH. Im going to change propellers and try that thanks for all the input.


Hi Dan I am located in Greenville South Carolina I usually go to lake Keowee and Jocassee. I appreciate the invite but I am on call with my Job this weekend. :mad: