Diameter

[TWPilot]

I've gone through 5 pages, and haven't truly found any post regarding diameter.

From the sticky:
1989 Glastron Sierra 160
Force 85hp
Original prop (handbook): 13.25x17
Original performance (handbook): 5k-5500rpm, 37-45mph

When I purchased this little boat (third owner), it had a 21p prop on it (not sure of diameter). Took it out last fall after purchase and prior to tear-down. Before putting it on the water, I changed plugs, rebuilt the carbs, and changed the foot oil. She would only do 4k rpms, and 23mph with a full tank.

Last winter I gutted the hull, replaced all the wood and added a larger tank (from 17gal to 32gal). Took it out with a full tank for the first time and the 21p prop. It still would only hit 4k and 23mph. So, checked timing and compression, rebuilt the fuel pump and link/sync'd the carbs. Engine fires right up, doesn't stumble, idles fine and when the throttle is punched- it doesn't hesitate. At WOT, she runs fine and doesn't seem to skip.

Thinking it was over prop'd- I purchased a 13.5x17 (what I found). Took it out and she gained 200rpm, and 4mph. No matter what I adjust on the motor, I think that's all she's got.

Long read- but would the extra .25" diameter have that great of an effect on the performance, much like a greater pitch (same concept, pushing more water)? I'm trying to achieve 5000rpm- and whatever speed that gives...

 

[Texasmark]

Going from 21 to 17 when the normal expected change, posted many times here on iboats, is 150-200 rpm per inch change in pitch says something is not telling you the truth. Diameter changes are part of the prop design and fractional inch changes are part of that. Pitch is king. Other thing is that when you change prop types/mfgrs, lots of things in the design of the prop can change so you really have to run it to know how it performs on your rig.

Trim position has a profound effect on top end. Getting the hull up and out of the water is a must...either by trim position/speed, or hull design. Glastron built a fine boat and I'd assume the Sierra 160 was a 16' fast runabout, mono hull, with some dead rise at the transom with lifting strakes along the hull fore to aft. See no reason why an 85 wouldn't get you up in at least the high 30's.

Has the boat been garage kept or left out in the weather with no covering over it's life? How about side and rear pictures, taken about 3' off the ground, 10-12' from the boat with your engine in it's normal tilt angle when running at WOT.

 

[ahicks]

This being a smaller boat, with a monster gas tank installed during a major rebuild, I'm wondering what this boat weighs? How close is it to the weight it was when it would run 40mph?

Concerned about the weight of that tank when filled, how it might be affecting the front to rear balance of the boat due to it's location, AND, the experience level of the person doing the hull rebuild. If that person built it like a battleship, it's possible there could be a LOT of extra weight there as well.

Last, wondering about the motor's compression. Thought there being It could be running great with compression that will never let it develop full power.

I do agree that trim issues (like motor height maybe?) could also be in play.

Last, I doubt prop diameter will have much to do with your issue. It's still nice to know more about it, so you might still want to read up on that.

edit: Another thought. Is the bottom of the boat straight? If it wasn't supported properly when the new stringers and floor were installed, there could be a deformation (a hook just ahead of the transom for instance) that will never let this boat go fast again.

 

[TWPilot]

Garage kept? Not sure- when I went to look at the boat, it was sitting outside, in the rain with no cover. It was pretty filled with leaves and pine needles, even in places you would think they couldn't get to. Tells me the guy I bought it from never covered it- which makes sense. He said he bought it for the summer and went out to fish and drink beer. The floor had two major soft spots in it, one back by the STBD rear seat (at one of the access ports) and in front of the ski-locker. There was a 4x6 block in the ski locker, probably to hold up the floor from rot- which it too, was rotted. I doubt it was garage kept, however the vinyl and brightwork looked good for a 30yo boat, and everything was there. No missing trim pieces, or broken bits.

Monster tank- I put in a larger tank because the 17gal only lasted about 3 hours prior to the tear-down, and I wanted the gas to last all day. Found out during the tear-down that the fuel pick-up line was leaking into the foam. That may have had something to do with the bad economy. As far as placement, the mid-section of the new tank is in the same position as the mid-section of the old. The front is about 6" fwd and the aft is about 6" aft of the original installation. The boat sits level in the water, and even with 1/4 tank, she still only gets up to 4200rpm and 27mph. Not sure about the weight, because I never weighed her before or after. Besides, even with the added fuel- that still only adds 90lbs, the weight of small person.

Built like a tank- not sure what the factory used for stringer material, but I used 3/4" marine ply, and in only factory locations. 2 layers of 1708 mesh covers all wood under the floor. The floor was made with 1/2" marine ply. 2oz foam was used under all voids, and the ski locker was kept. While under construction, the hull was supported by the trailer rollers, and longitudinal beams underneath. I don't think there's a hook, the beams kept the bottom straight during the process and not removed until after I was done.

Compression- with my cheap harbor freight gauge, all three cylinders read 105psi. It's low, but all three are reading the same. I doubt all three would be low at the same rate, but I could be wrong as well.

 

[Scott Danforth]

a 6' straight-edge will let you know if you have a hook or rocker in the hull
borrowing a good pressure gauge will verify if you are truly low on compression........ takes squish to make bang
if you truly are low on compression, you no longer have 85hp.
the extra 15 gallons of fuel and larger tank added about 120#