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75hp Merc 2-stroke on a 16.
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| Fishmore |
Posted on 08/18/08 - 5:25 PM
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Well I finally got to do a fishing trip with my new to me 75hp 1994 Mercury 2-stroke on my 73 re-built Montauk-style hull. Engine ran great, Jumped up on plane and cruised nicely at 3800rpm at about 28mph. WOT was 36mph at 5400 RPM. troll was 2.2 mph at 750 rpm and 3.5mph at 1000 rpm .
Fuel economy was beyond my expectations. In general I did a combination of running and trolling covered about 32 miles in 5.5 hours and burned 4.26 gals of fuel. Running time was 32 minutes/14 miles the rest was spent trolling. Engine was off for about 30 minutes total but at random times during the day. People and gear weighed about 700 lbs. All speeds and distances by GPS. Fuel by refilling the tank.
I have not recieved my manuals yet (on order) but I think that WOT on this motor should be 5250 RPM and according to my research top speed should be closer to 40mph. I am currently using the original 17p aluminum prop. I had some time this afternoon so I went to the Mercury site and found/ran thier prop selector inputing my performance settings and they reccomended 4 props for overall performance.
Black Max Aluminum 3-blade 13x19
LaserII Stainless 3-blade 13.25x19
Trophy+ Stainless 4-blade 13.75x19
Vengeance Stainless 3-blade 13x18
Interesting to note was that if I changed my criteria to improved acceleration the LaserII was dropeed from the reccomendation and all props were dropped to a 17p. If I changed criteria to improved top speed then the Black Max was dropped and the Trophy+ dropped pitch to 17.
Does anyone have experience with the previously mentioned props or have recomendations? I am leaning towards the LaserII in 19p or the Vengeance in 18p. I am one-hole up and my cavitation plate sits 1.25 above the bottom of the transom.
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| joninnj |
Posted on 08/18/08 - 7:34 PM
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Hey Fishmore,
Glad its running well for you... 
I think any of the 19's will be a bit to tall for your set up. I run a 19 inch on a 17 ft with a 90 HP. I squeeze 5200 RPM out of a 13 X 19 K series SS Yamaha prop. The gear ratio is 2:1. Size of the blades with effect how it run. 13 vs 13.5 diameter will allow slightly more pitch, meaning smaller blade-more pitch...
My guess (only a guess) is a 17 may be better for you and that might even be a bit big... In a like for like scenario you could loose RPM with a stainless, but will be able to trim out and not break loose of the surface. can't really do that with an aluminum.
The Merc guide on only that, a guide. When I run it tells me I need a 20.... I know that is to big... Engine height also plays a factor.
BTW the Laser is great prop. I used one on another boat, all around great performance.
Is there a way to test them? That is always the best way, but not always available (almost never...) 
Edited by joninnj on 08/18/08 - 8:01 PM
Jon in NJ
Many other boats and outboards in my boating history
The Whaler is the one I like the best!!! |
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| Fishmore |
Posted on 08/21/08 - 2:04 AM
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Thanks Jon, I have found out that my engine has a 2.33:1 gear ratio and I tried two other prop selectors that also recomended a 19 pitch prop. I could not find a shop that would loan me a prop so I have purchased a 13.25 x 19 Aluminum one just to see if the engine can handle that size prop. I can always keep it for a spare.
By the way my local dealer and some other people feel that I am just wasting my time. I hope not.
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| nh5 |
Posted on 08/21/08 - 7:20 AM
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I got the same engine as yours except mine might be a 95 I can't really confirm which year it is. But WOT is 5250 as far as my manual says. I got the Laser II on my whaler. At WOT I get to like 5300RPM and like 42mph. I do have a bit smaller boat. Mine does amazing on gas just up on plain going about 28 also. Otherwise it chugs gas.
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| Fishmore |
Posted on 08/21/08 - 7:01 PM
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Good info LH, what pitch is your Laser prop?
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| nh5 |
Posted on 08/21/08 - 7:31 PM
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I couldn't tell you at all since im not the original owner of the boat and I have no clue how to find out the pitch of the prop. Something i've never looked into
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| Fishmore |
Posted on 08/21/08 - 8:21 PM
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The diameter and pitch is usually embossed on the prop but it is on the backside in most instances and you need to remove the prop to see it. My current Aluminum has the imprint on the outside of the prop. I remove mine at least once a year to clean it and the shaft up check for fishing line etc... and grease the shaft. If you do not grease the shaft under the prop the prop can get stuck on the shaft and then can be real hard to get off requiring a puller and heat which can damage the prop. If you get fishing line on the shaft it can cut into the rear seal and cause it to leak which can ruin your lower unit or at least the bearings in the lower unit. I am not saying you need to do it right away but it is a good thing to do during your fall/winter maintenance.
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| Derwd24 |
Posted on 08/21/08 - 11:04 PM
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On the Mercury Mirage, the pitch is stamped right on the housing in between the blades. I'd think the Laser II would be similar?
Dave - 1983 Outrage 22 |
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| nh5 |
Posted on 08/22/08 - 7:12 AM
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Hmm I never knew about this. I'm gonna do it when I have time and i'll check the pitch.
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| Fishmore |
Posted on 08/29/08 - 12:20 AM
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I did a test tonight with the new prop. I installed a 13.25 x 19 in place of the 17 pitch I had on my 94 Mercury 75hp 2-stroke.
Old 17p AL prop was running
3800rpm at 28mph. WOT was 36mph at 5400
New 19p AL prop was running
3800rpm at 29.5mph WOT was 38.5 at 5400
So I went up 2' in pitch and the WOT RPM stayed the same so there must be a rev limiter on this engine.
By the way even after going up in pitch my time to plane was the same. I mean you hit the throttle and you are on plane no waiting to get over the hump or anything it just goes.
I am not sure what I am going to do now. I am trying to get the motor to max out at the reccomended maximum RPM of 5250.
Either I buy a SS prop that is 13.25 x 19 or I buy a 21 pitch AL and try that.
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| Tom W Clark |
Posted on 08/29/08 - 8:10 AM
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Fishmore,
There is no rev limiter on the Mercury 75. The reason you saw no difference between the two props is probably because there are not the same model of prop or one had its pitch changed.
A two stroke Mercury 75 on a 16 foot Whaler should be spinning a 13" x 18" Mercury Vengeance of a 13-1/4" x 17" Stiletto Advantage 4.25.
Edited by Tom W Clark on 08/29/08 - 8:11 AM |
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| John Fyke |
Posted on 08/29/08 - 10:57 AM
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[quote]Fishmore wrote:
I have not recieved my manuals yet (on order) but I think that WOT on this motor should be 5250 RPM
I have a 1987 70hp and my WOT RPM's is 5250 so you might be right.
John Fyke
Re-Fit or Reef It
1979 15' Sport with Super Sport conversion and 70hp mercury. |
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| Fishmore |
Posted on 08/29/08 - 10:11 PM
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Thanks John, yes I verified in my shop manual that it should run between 4750 and 5250 rpm at WOT.
Thanks Tom I will stop playing with the Al props and order the Stilletto.
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| nh5 |
Posted on 08/30/08 - 9:54 AM
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Alright I took my prop off and greased checked for any garbage stuck in there. I read this info off the side: 48-16992 A40 22P. Guessing 22P is 22 Pitch
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| Fishmore |
Posted on 08/31/08 - 10:08 AM
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NH, wow a 22 pitch SS Laser on that motor and you are right at 5300. How is the acceleration? Does it jump on to plane or is it a little slow getting over the hump? Do you do any watersports pulling tubes, skiers, etc...)? I know the 15' Dauntless is a very different hull from my 16 Montauk but the Dauntless tended to have a deeper V so I would think it would be harder to get on plane but might be faster then the 16 and it might weigh about the same.
After running the 19 pitch Al prop I was thinking I could go to 21 pitch without too much trouble but I was not sure.
Tom based on NH's performance would you still stick with the 17p Stilleto or go to the 19p?
Edited by Fishmore on 08/31/08 - 10:14 AM |
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| nh5 |
Posted on 09/01/08 - 2:57 PM
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I have not been out on the water at all this year yet due to the pate tank causing havoc this year. Waiting for my replacement tank which will get put in by next weekend. Maybe I'll have some time to get out one weekend. It's got new carb kits and some other items so it might even go faster I'll time it on plane getting up on plane. But as far as I remember it jumps up. I do pull tubes and probably skiers soon.... Can't wait to get out at least once this year lol. At least next season should be problem free.
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| Fishmore |
Posted on 09/28/08 - 1:57 AM
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Got to test the reworked Stiletto today. Tim at RTK repitched it to 19. Results are that the prop has my RPMs at the perfect setting now 5200 at WOT. Manufacturers specs call for optimum rpm at WOT of 4750 to 5250 so I would say you can not get much better than 5200!
Final results are:
4000 RPM 27.5 mph
5200 RPM 38.5 mph (WOT)
In addition, I still get to 2.0 mph at idle and if I trim the motor all the way in I can plane at 16 mph. By the way it also still hops the boat right up on plane.
I can highly recomend Tim at RTK in San Leandro for your prop needs. The prop does exactly what I asked for, the price was reasonable (much better than buying another prop) and the prop was ready on the day it was promised to me. Perfect!
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