performance question

[2cycle]

I have a 74 150 HP on a 73 18 ft. Glastron. It has a 15 pitch prop that turns 5400 RPM at WOT. I'm not sure of the boat's weight, but is doesn't seem obscenely heavy. I have a Hydrofoil also and am hitting about 36 MPH. The cavitation plate is even with the bottom of the transom. The compression is between 110-120 PSI in all 6 cylinders. Does the performance sound about right or should I be getting more out of the engine/boat setup? I was a little surprised that I had to use a 15 to get closer to the specified RPM range with a motor that size. Is there anything I could be missing in maybe getting the boat closer to the 40 MPH range (I have not ran it without the Hydrofoil)? I appreciate any suggestions/advise. Thanks.

 

[ZmOz]

Re: performance question

Yes, it does sound like you should be faster. I have the same motor on a '72 16 foot Glastron. It goes 51 mph GPS with a 19p at about 6100 RPMs, I don't think 60mph is out of the question with the right stainless prop. Have you got it trimmed right? How about the spark plugs...are they clean? Has it been decarbed?

 

[2cycle]

Re: performance question

I de-carbed it with SeaFoam and the compression improved about 5 lbs. in each cylinder. I probably should replace the spark plugs, but there is no skipping or missing at WOT. Re: the trim, I'm not sure, but I did follow the Clymer manual for setting the trim limit adjustment and it stops right at the outer edge of the bracket. Could the hydrofoil be slowing it down? Next time I take it out I guess I could try and trim it up higher with the up/out switch. The prop seems OK with no major dings. It sounds like your boat really moves (Wow!). Is your timing at 21?

 

[ZmOz]

Re: performance question

Yes, you should be playing with the trim while you're out. When it's setup for top speed, you can't really get going without cavitation, so you need to adjust it while you're moving. This is with an aluminum prop right? Are you alone in the boat, or is there other people/lots of gas? That can make a big difference too. To tell you the truth, I don't know where the timing was set at...but it's probably been there for a while without problems...

 

[ZmOz]

Re: performance question

Also - I don't think the hydrofoil will hurt it more than 1-2mph, but you might want to try taking it off and see how it is.

 

[Spidybot]

Re: performance question

Agreed. When you look for speed optimization it is important to know and minimize your weight. It's by far the best performance per $. A tach is needed to know how it turns out.<br /><br />Browsing my archive I find a Glastron 1979 brochure that shows a range of open bow models in your approx. size. Named 184 to 189 it indicates various models of 18' - all tri-hulls. The SSV 188 is rated for 150 hp max. and should do better than 36 mph. Those days spec given were:<br /><br />Lenght 17' 11½"<br />Beam 88"<br />Transom width 84 3/4"<br />Depth 45 3/4"<br />Motor shaft length 25"<br />Capacity 1200 lbs<br />Tank 25 gallon<br /><br />Plus some info on equipment and flotation but no info on GW (?)<br /><br />In a book from 1970 I find a 'Swinger V 176' of similar design. Rated for 140 hp max. and a GW of 880 lbs (dry, excl. motor). Max speed stated as 42 knots.

 

[2cycle]

Re: performance question

Thanks a lot for the reply. The prop is aluminum and thus far I've been by myself when taking it out with usually no more than 16 gallons of fuel on board. Also, looking at the transom and design of the boat, it looks almost like the back of the boat slopes inward a little bit from top to bottom and I wonder if I just need to trim it higher once I get up to speed to make up for this? The manual said to set the trim limit adjustment switch about 1/2 inch from the top adjustment pin/holes. I will take it out and play with it a little bit and hopefully be able to report back with some tangible gains. I really like the motor and it sounds good when it gets wound out. What does yours idle at in FWD gear? I can't get much under 1000 without it wanting to die. Thanks again.

 

[ZmOz]

Re: performance question

I seem to recal mine idling pretty low in gear...like around 500. One more thing - are you going by the boat's speedometer? They are almost allways incorrect, mine is off by about 10mph.

 

[2cycle]

Re: performance question

UU,<br /> Thanks for looking up those specs--very informative. Mine is sort of a hybrid V/Tri-Hull (somewhat of a flat bottom but not as flat as some/most tr-hulls). It looks like I need to find some missing speed somewhere... Thanks.

 

[Spidybot]

Re: performance question

Your idle sounds ok.<br /><br />The mentioined brochure shows the transom design as you describe it. The actual angle of the motor mount is not different from most.<br /><br />Power trim is nice but even if you're setting it by moving the pin you'll experience big differences. As trim will give you bow lift it'll take the hull out of the water and make a huge difference in the resistance that your motor have to overcome.<br /><br />See<br /> http://www.spidybot.com/prop/prop_049.htm

 

[2cycle]

Re: performance question

ZmOz,<br /> Yes, I'm going by the newly installed fauria speedometer and pickup tube. It seems fairly accurate in terms of other boats I've been in by comparison--but like you said it may be off. <br />Your idle is a lot closer to the specs than mine (just worried about putting too much stress on shifting into gear). Thanks.

 

[Chris1956]

Re: performance question

I also think you should be going faster. I have a '77 150HP Merc on a 16 foot Avenger. I get 60+ MPH when the motor operates at top efficiency. First, I would make sure that all cylinders are firing. If so, run WOT and trim the motor until the boat starts to porpoise or the motor starts to ventilate. The steering should pull left at this point. Then trim down slightly until the ventilation or porpoising cease. This should be optimum. I also thing a 19-21P prop is probably the correct one for general use. If the speed does not increase, check to see if your hull has a hook or other defect. This will cause the hull to plow water.<br />PS at WOT and zero trim my boat goes about 45MPH, at full trim the speed increases to about 62MPH

 

[quantumleap]

Re: performance question

With a 15 pitch prop your top speed will be much lower. A 19-21 wil bring your top speed up. Are you sure your tach is accurate? Also ,the "newly installed" could be a problem. See if you can borrow a GPS to check your speedo.

 

[2cycle]

Re: performance question

I think the tachometer is working properly. It's a new Quicksilver. I connected the sender pole to one leg of the stator and set the arrow on the back of the gauge to the 12 pole setting. The motor came with a 19 inch prop, but I was concerned that it would not get me into the right RPM range @ WOT, and I did not want to make it lug and detonate--so I tried the 15. I have since picked up a 17 as well, so I guess I can experiment with the trim limit and prop size when I get out on the water next. I don't see any structural problems on the underside of the boat. I appreciate all the helpful suggestions and will update further after I take it out. It's also very interesting to hear the performance that others are getting from the motor with other boats. That's a lot of bang for the buck for sure.. Thanks.

 

[2cycle]

Re: performance question

Update,<br /> I took the boat out and tried a 19 pitch prop along with the 15. The 15 has the numbers 15P on the outside of the prop and, as earlier posts note, performs well with the holeshot putting the WOT RPMs at 5400-5500 at about 35MPH. The hub is solid without the locking nut with splines that goes over some of these in front of the lock washer and nut The 15 just slides on with the lockwasher and nut to follow. The 19 has "19" engraved in the hub and slides on the splines, followed by a washer with splines and then the lockwasher and nut. The reason I mention this is because the 19 wanted to rev out of the hole and ran right at 5400 @ 35 MPH just like the 15--but this can't be right?? The 19 did take longer to plane but did not produce performance gains at WOT that I expected. In fact, both the 15 and 19 cruise at 4200 RPMs at 30 MPH. The props look different and I'm reasonable sure that they are what they are, but something is not adding up. Could the rev mean that the 19 prop hub is having some slippage? I'm at a loss. Any ideas or diagnoses are welcomed and appreciated. I know I've got to be missing something.. Thanks.

 

[2cycle]

Re: performance question

Here is a link to a visual of the boat for any visual learners out there..<br /><br /> http://www.classicglastron.com/73gl-v187-186.html <br /><br />Mine is the V186 (only it does not look vintage but has no discernible hull flaws that I can tell)

 

[quantumleap]

Re: performance question

Something is not adding up there! Either you are mistaken about the prop pitch on one or the other, or the hub on the 19 is slipping, or one of them was repitched. Are they both new or used? There should be s significant different in rpms at WOT between a 15 and a 19.

 

[2cycle]

Re: performance question

Both of the props are used but definitely look different from a glance. I'll have to measure but the 15 looks like it has a slightly larger diameter also. Also, the 19 is vented with 2 holes in front of each blade. Could this account for slippage or spinning out of the hole to keep RPMs up? I also took the hydrofoil off and that really did not make much of a difference other than the boat coming up a little more in the front when planing (no real differences in WOT MPH). I hate to have to start buying props to experiment, but I may have to I guess. Thanks for the response.