Spectra 171 - How Fast?

[11YearItch]

So far I have been able to get 45 MPH at 5000 RPM from the 1988 Spectra 171 I purchased this past summer. That speed came with one person in the boat and a half a tank (10 gallons) of fuel. The boat has trim tabs, a 1988 Evinrude XP 150 V6 outboard engine with power trim, and no jack plate. The prop is a three blade 14.5 x 19.

Is 45 MPH about right for the boat as described or should it be higher? What is the maximum speed I can expect if I make changes to the setup, i.e. jack plate, different prop, etc.?

 

[paulsboats]

RPMs sound low for that prop.

 

[11YearItch]

What should the RPM's be and how do I get there with the existing prop?

 

[Jimway]

Sounds about right with a 1.8 or so gear ratio in the lower unit? An extra 500 rpm might net you 4 mph.

 

[rsukup]

I put your numbers in the prop calculator using 20 for a cupped prop and got 12.5%. Sounds about right. Boat weighs 1100. My question is. Do trim tabs slow down the boat? I just raised mine because I think they drag with the motor trimmed. I'm getting too much slip with a Carlson CVX 16. What rpm are others with your boat getting? Sounds like 1 cyl may be out.
Read this post:
http://checkmate-boats.com/forums/showthread.php?t=24581&highlight=spectra

 

[freightguy]

I havent had my boat that long to do many speed test but I did get 49 MPH on gps only half trimmed up. The speedometer was showing 55 at that time.

I think I could get a true 55 out of it. I dont know what my prop size is. I keep thinking 27, I think its the original prop.

 

[11YearItch]

I put your numbers in the prop calculator using 20 for a cupped prop and got 12.5%. Sounds about right. Boat weighs 1100. My question is. Do trim tabs slow down the boat? I just raised mine because I think they drag with the motor trimmed. I'm getting too much slip with a Carlson CVX 16. What rpm are others with your boat getting? Sounds like 1 cyl may be out.
Read this post:
http://checkmate-boats.com/forums/showthread.php?t=24581&highlight=spectra

rsukup, I don't know if trim tabs slow down the boat. When I purchased my boat it had a whale tail. I didn't like the way my boat came up out of the hole (too much bow rise) and how it performed at 30-40 mph (some chime walk) so I removed the whale tail and installed the Nauticus automatic operation smart tabs. I am very happy with the tabs because they resolved both of my problems. Also, I am ok with my boat doing only 45 mph as long as it runs smoothly at 30-35 mph which it does.

As for your comment about one cyl being out I don't think this is the case. When I winterized the boat I pulled and looked at all six plugs and they all looked the same, i.e. there wasn't one plug that showed signs of being a problem.

 

[11YearItch]

I havent had my boat that long to do many speed test but I did get 49 MPH on gps only half trimmed up. The speedometer was showing 55 at that time.

I think I could get a true 55 out of it. I dont know what my prop size is. I keep thinking 27, I think its the original prop.

freightguy, thanks for your response to my speed question. I also haven't had my boat very long but am just curious what it's maximum speed should be. I checked my speedometer vs gps at 30 and 35 mph and found it to dead on so am assuming it is probably also accurate at 45 mph.

As for prop size my prop is a three blade 14.5 x 19 where 14.5 is the diameter of the prop and 19 is the pitch. I believe my prop is also the original that came with my Evinrude 150 hp v6.

 

[JW]

My buddy that has a Spectra 170 with Merc XR4 is running low sixties using a 24" Laser II. No bow cover, no setback, all stock from the day it left the showroom. We are at 950' altitude. His LU ratio is 1.78:1 and is the XR smaller diameter

 

[11YearItch]

My buddy that has a Spectra 170 with Merc XR4 is running low sixties using a 24" Laser II. No bow cover, no setback, all stock from the day it left the showroom. We are at 950' altitude. His LU ratio is 1.78:1 and is the XR smaller diameter

Thanks JW for the info. I find it interesting that by simply changing the prop I might be able to add as much as 15 mph.

 

[JW]

Thanks JW for the info. I find it interesting that by simply changing the prop I might be able to add as much as 15 mph.

Probably not just a prop change. You'll have to do a lot of mods to gain that kind of speed. Depends on how much power your engine is really putting out, and whether or not your hull has developed a hook over the years.

For fun, you can take a 4' straight edge to your pad and see how flat it is from the back of the transom forward. There will most likely be a little hook, possibly 1/8" over the length of the straight edge. The more hook you have, the worse top end you'll have. If your hull has a rocker, then it'll porpoise at speed.

If your bottom is soft, you could be experiencing hook from water pressure on the bottom of the boat at speed that you cannot see on the trailer. The way to test that is to use a suitable block of wood as protection under the pad and carefully jack up the boat using a floor jack, maybe 3 feet from the transom. I'm not saying off the bunks, just up a little. If you carefully lift the boat, and your straightedge doesn't show any more hook, then your bottom is probably still strong. If you put pressure on the pad with a jack and the hook increases, then your bottom is soft and ready to come apart. This is an important test to do to older CMates before buying, but it sure freaks out the seller.

Back to real engine power. The XR motors had more power from the factory than advertised. That being said, so did many of the JohnnyRude motors. The Mercs just had a little bit more .
Your gearcase is another factor. The Merc XR gear cases, with the smaller diameter, go through the water faster than a standard sized gear case when you run them buried.

Keep trying props until you find the 'best' one for your use. And if you can get that boat to 55, then install a tight fitting bow cover to gain a few more 'free' mph on top end.

 

[11YearItch]

Probably not just a prop change. You'll have to do a lot of mods to gain that kind of speed. Depends on how much power your engine is really putting out, and whether or not your hull has developed a hook over the years.

For fun, you can take a 4' straight edge to your pad and see how flat it is from the back of the transom forward. There will most likely be a little hook, possibly 1/8" over the length of the straight edge. The more hook you have, the worse top end you'll have. If your hull has a rocker, then it'll porpoise at speed.

If your bottom is soft, you could be experiencing hook from water pressure on the bottom of the boat at speed that you cannot see on the trailer. The way to test that is to use a suitable block of wood as protection under the pad and carefully jack up the boat using a floor jack, maybe 3 feet from the transom. I'm not saying off the bunks, just up a little. If you carefully lift the boat, and your straightedge doesn't show any more hook, then your bottom is probably still strong. If you put pressure on the pad with a jack and the hook increases, then your bottom is soft and ready to come apart. This is an important test to do to older CMates before buying, but it sure freaks out the seller.

Back to real engine power. The XR motors had more power from the factory than advertised. That being said, so did many of the JohnnyRude motors. The Mercs just had a little bit more .
Your gearcase is another factor. The Merc XR gear cases, with the smaller diameter, go through the water faster than a standard sized gear case when you run them buried.

Keep trying props until you find the 'best' one for your use. And if you can get that boat to 55, then install a tight fitting bow cover to gain a few more 'free' mph on top end.

JW, When I purchased my boat it had a whale tail. I didn't like the way the boat came up out of the hole (too much bow rise) and how it performed at 30-40 mph (some chine walk) so I removed the whale tail and installed the Nauticus automatic operation smart tabs. I am very happy because the tabs resolved both of my problems. Also, I did experience some porpoise when the whale tail was installed but not since installing the trim tabs.

Next Spring before putting my boat back in the water I think, just for fun, I will perform your suggested 4' straight edge test for hook. Hopefully the hook will be minimal. Thanks for your input.

 

[rsukup]

Spectra 171

When you get to where you can work on your engine check each cylinder for 7/16" spark with a spark tester. Also check compression with a good tester. Cold it will run about 10 lbs low. Don't know what the numbers should be with the V-6 but check other forums like Marineengine.com. That XP should be comparable to the Merc XR. See if you can verify any of this:
Dyno specs for 83-91 150XP 2800 psi
84-91 OMC 150 3050
86-88 Merc XR2 3300
Was surprised to see this but that's what was listed. You can spend your free time searching to see if the Merc XR or the Evinrude XP is more powerful. The 150HP should run higher RPM than 5000 with a 19" prop.

 

[11YearItch]

When you get to where you can work on your engine check each cylinder for 7/16" spark with a spark tester. Also check compression with a good tester. Cold it will run about 10 lbs low. Don't know what the numbers should be with the V-6 but check other forums like Marineengine.com. That XP should be comparable to the Merc XR. See if you can verify any of this:
Dyno specs for 83-91 150XP 2800 psi
84-91 OMC 150 3050
86-88 Merc XR2 3300
Was surprised to see this but that's what was listed. You can spend your free time searching to see if the Merc XR or the Evinrude XP is more powerful. The 150HP should run higher RPM than 5000 with a 19" prop.

Thanks rsukup for the good information. I too am surprised at how comparable my XP is to the Merc XR.

When I purchased my Spectra this past summer I was told the engine only had about twenty hours on it since being rebuilt. I was given a thirty day warranty so shortly after taking ownership I had a compression test run. All cylinders were at 110 lbs. I think this test was done on a warm engine.

I won't be able to work on the boat or engine for a few months because it is now tarp covered in my yard for the winter. When Spring comes I plan to run another compression test myself and at the same time will take your advice and check each cylinder with a spark tester. I will continue to research why I can't get about 5800 rpm from the engine that would then give me about 50 mph.

 

[rsukup]

Another member posted that the HP of the XP is under rated at 150 vs the standard 150. I'm guessing the Dyno tests were recorded in reverse. I can't find the site now but there are other misinterpretations. Lower unit ratio will change the torque specs as well as rpm may be less at 6000 than 5500 for example. 110 sounds about right . Heard some 150s read 105. I have a Sun guage and get about 125-130 on an 87 Johnson 140. 110 -120 cold.

 

[Checker7]

Thanks JW for the info. I find it interesting that by simply changing the prop I might be able to add as much as 15 mph.

Doubt you'll get that much from a prop change unless your current one is junk.

A 19 should be over revving on that boat and motor.

I'm getting 55 mph, loaded down with gear and two ppl at 5500 or so, with
Merc 150 XR4 and 22p Laser II three blade Stainless.