HP required to plane 12ft Higher BBSB.

I should clarify that while on the road I would carry the 8hp in the cockpit of my Higbee or in my SUV, placing the 8hp on transom before launch. So the long bumpy road would be less ofoa concern. Whereas the 9.9/15 is a bit too heavy for my aging lower back to handle this on/off routine. I could also lock the 8hp in my vehicle when needed.
 
The difference in weight between my 8 hp merc and 15 hp merc 2 strokes is 3 lbs. 9 hp 2 stroke merc same as 15. Both 8 & 15 are 90,s versions.
 
Roy, thanks for input. And I'll apologize in advance to All for a long post.

On the Mercury outboards with only a 3 pound difference between 8hp and 15hp I too would definitely use the 15 instead of 8. What are the actual weights (or Model/serialNumbers--Mercury doesn't use Models, right?) of the 8 and 15 (with the 9hp being same weight as 15 in Mercury product line) ? It seems to me that the Mercury 8 is either quite heavy or the Mercury 15 is quite lightweight-- or maybe some of both.

I prefer Johnson/Evinrudes for 2 reasons (and admittedly maybe these are not really great reasons. Perhaps I need to "open my mind" a bit.). First reason is that I have been tinkering with OMC products for around 10 years, so that's where my hands-on experience (albeit limited) exists. Secondly, I have had trouble finding a knowledgeable and fairly-priced mechanic for small (< 40hp) outboards in our small town. I have found one good small outboard mechanic and he strongly prefers OMC's (and Yamahas). He claims that "Mercury's just blow up" and Suzuki's self-destruct via corrosion. Although I have friends with larger (>40hp) Mercurys) and their experiences have been no worse than my experience with my 90HP 2000 Johnson, I am inclined to stay with OMC's based on my mechanic's advice.

So...here are the weights that I know for OMC's. I have a 1969 6hp Model 6R69 Johnson (runs great!) listed Weight on iboats website is 51# (I weighed it at 54# --so there is a 3 lbs discrepancy). I am considering purchase of a 1985 Evinrude 8RCO which iboats lists as 56# (and Sellers have weighed at 56 to 58#). I am also considering purchase of a 1986 9.9 Johnson 10RCD which iboats lists as 72#. iboats also lists 1986 15HP Johnson 15RC as 72#. So in the Johnson/Evinrude product line there is a 16# difference between 8HP and 9.9/15hp. This makes sense as I understand only difference 9.9 vs. 15 is carburetor.
That 16# difference seems significant when I am hanging it on my Higbee's transom, or lifting it with my aging lower back (I spent a year or so getting chiropractor fix on my back not long ago).

A couple years ago I bought a really sweet used Mercury 6hp (s/n was either 3372456 or J3061988--my records are unclear). It weighed a whopping 70 lbs! This is compared to 54# on my 1969 Johnson 6hp 6R69M. But if a Mercury 9.9 or 15hp weighed only 73lbs (=70 +3) then that would be comparable to OMC weights.

I will do some more searching on websites for Mercury 9.9/15 weights--maybe I am missing some good deals on a used Merc.

Thanks again for all inputs.
 
Guessing that 6 hp merc you bought was made on 8hp block. Merc has a tendency to do that within their old small 2 strokes. I,ll check the year on my 15 for you .
 
David Clites said:
Tim, thanks for the input. I am working on buying a 15hp Johnson or Evinrude 2 stroke (or at least a 9.9 that I can convert by changing the carburetor).
Click to expand...

David, Just a point of clarification concerning the OMC 9.9 to 15 hp. conversion. Changing the carb alone will not be a complete conversion and will not produce 15 hp.

The inner exhaust housing MUST be swapped out for the one designed for the 15hp engines. ( one can look at the parts list for both engines and see that these parts are in fact different)

Failure to make BOTH modifications will result in less than the desired HP increase.
 
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Let me preface this input, I own and run a 15hp Yamaha, and am very, very happy with it. If you are running a sneakbox, with a tiller extension, the Mercury offers a very handy feature with the F-N-R in the twist handle. Not having to reach back to the motor to shift is very handy, though it takes some getting used to. My buddy has this on his TDB, and I sometimes will run it to chase a cripple when we park the boat away from our spread, simply because I am faster to the boat than he can be...... IDK if any other manufacturers offer this, but I like it. I have noodled a remote shift modification for my Yamaha, but have not tried to implement it yet.
 
David Clites said:
Roy, thanks for input. And I'll apologize in advance to All for a long post.

On the Mercury outboards with only a 3 pound difference between 8hp and 15hp I too would definitely use the 15 instead of 8. What are the actual weights (or Model/serialNumbers--Mercury doesn't use Models, right?) of the 8 and 15 (with the 9hp being same weight as 15 in Mercury product line) ? It seems to me that the Mercury 8 is either quite heavy or the Mercury 15 is quite lightweight-- or maybe some of both.

I prefer Johnson/Evinrudes for 2 reasons (and admittedly maybe these are not really great reasons. Perhaps I need to "open my mind" a bit.). First reason is that I have been tinkering with OMC products for around 10 years, so that's where my hands-on experience (albeit limited) exists. Secondly, I have had trouble finding a knowledgeable and fairly-priced mechanic for small (< 40hp) outboards in our small town. I have found one good small outboard mechanic and he strongly prefers OMC's (and Yamahas). He claims that "Mercury's just blow up" and Suzuki's self-destruct via corrosion. Although I have friends with larger (>40hp) Mercurys) and their experiences have been no worse than my experience with my 90HP 2000 Johnson, I am inclined to stay with OMC's based on my mechanic's advice.

So...here are the weights that I know for OMC's. I have a 1969 6hp Model 6R69 Johnson (runs great!) listed Weight on iboats website is 51# (I weighed it at 54# --so there is a 3 lbs discrepancy). I am considering purchase of a 1985 Evinrude 8RCO which iboats lists as 56# (and Sellers have weighed at 56 to 58#). I am also considering purchase of a 1986 9.9 Johnson 10RCD which iboats lists as 72#. iboats also lists 1986 15HP Johnson 15RC as 72#. So in the Johnson/Evinrude product line there is a 16# difference between 8HP and 9.9/15hp. This makes sense as I understand only difference 9.9 vs. 15 is carburetor.
That 16# difference seems significant when I am hanging it on my Higbee's transom, or lifting it with my aging lower back (I spent a year or so getting chiropractor fix on my back not long ago).

A couple years ago I bought a really sweet used Mercury 6hp (s/n was either 3372456 or J3061988--my records are unclear). It weighed a whopping 70 lbs! This is compared to 54# on my 1969 Johnson 6hp 6R69M. But if a Mercury 9.9 or 15hp weighed only 73lbs (=70 +3) then that would be comparable to OMC weights.

I will do some more searching on websites for Mercury 9.9/15 weights--maybe I am missing some good deals on a used Merc.
Dave my 8hp merc is a 1999 sticker weight of 73 lbs. My 15hp merc is 2004 and sticker weight of 76 lbs. Both short shaft tiller motors. 9 hp in 2004 model year should be same weight as 15 hp. I do know the 8 can,t be bumped to 15 by changing carbs, etc. but the 9 in 2004 could be.
Thanks again for all inputs.
Click to expand...
 
HuntinDave, thanks for clarification. It makes sense that exhaust needs to be modified too. I have seen similar situations on motorcycles. I know someone who has a converted 9.9 to 15 and he is disappointed with results. I will ask him if exhaust not just carb was changed.
 
Has anybody on this forum performed a 9.9 to 15hp Johnson 2stroke conversion?

More specifically does anybody know what the correct Carburetor p/n(s) is/are for a 1986 Johnson conversion 9.9hp to 15hp? On multiple vendor websites I see the same Carburetor Assembly p/n's listed between the 9.9 and 15hp. However I do see the Inner Exhaust Housing is different (p/n 391578 for 9.9hp, 330439 for 15hp). Good news is there are plenty of Inner Exhaust Housings available for $12-15 on eBay. Bad news is the engine I am thinking to buy might have powerhead (crankshaft) frozen to drive shaft (a very common occurance on Saltwater engines) and therefore might prevent me from swapping out the Exhaust Housing.

If the carburetors are same between 9.9 and 15 then maybe I need to do only the Exhaust swapout???
 
Dave, got to tell you that you would never see the difference in that boat weather it's a 9.9 or a 15 Johnson or Evinrude. MAY BE IF you are running it with NOTHING at all in the boat( no gun,no decoys, no dog,) I mean nothing. The only way you can get MAYBE 2 more miles per hr is to go to a 15 2 stroke Yamaha and at that you will need to try 3 different pitched props. If you use anything other than a OMC prop it will not preform as well. Every part on 15 and 9.9 OMC is interchangeable.
Phil
 
David Clites said:
Has anybody on this forum performed a 9.9 to 15hp Johnson 2stroke conversion?

More specifically does anybody know what the correct Carburetor p/n(s) is/are for a 1986 Johnson conversion 9.9hp to 15hp? On multiple vendor websites I see the same Carburetor Assembly p/n's listed between the 9.9 and 15hp. However I do see the Inner Exhaust Housing is different (p/n 391578 for 9.9hp, 330439 for 15hp). Good news is there are plenty of Inner Exhaust Housings available for $12-15 on eBay. Bad news is the engine I am thinking to buy might have powerhead (crankshaft) frozen to drive shaft (a very common occurance on Saltwater engines) and therefore might prevent me from swapping out the Exhaust Housing.

If the carburetors are same between 9.9 and 15 then maybe I need to do only the Exhaust swapout???
Click to expand...

Not sure where you are getting your information but the info I'm seeing shows different carburetor assembly part numbers for these engines. The jet sizes are different also but I don't know if the bore size between the carbs is different or not.
[strike][/strike]
1986 Johnson Outboards 9.9[strike][/strike]
0434139 ---- 0395915 9.9 Models Only
0436991 ---- 0395912 15 Models Only [strike][/strike]
 
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Best place to look up part numbers is the factory BRP web site. link to BRP
https://www.brp.com/
0330440 EXHAUST TUBE, Inner. 15 Models Only 1 $100.89 USD
stock_high.gif
Available [strike][/strike]
[strike][/strike]
0323151 ORIFICE, PLUG, No. 45. 9.9 Models Only [strike][/strike]
0326858 ORIFICE, PLUG, No. 45. 15 Models Only [strike][/strike]
[strike][/strike]
0395915 0395915 CARBURETOR ASSEMBLY. Not Shown 9.9 Models Only

[/url]
0395912 0395912
CARBURETOR ASSEMBLY. Not Shown 15 Models Only
 
Thanks to all for the input. I bought a 1986 Johnson 9.9 over past weekend. It runs ok but may have some screws rusted in Midsection that could prevent access to Inner Exhaust Housing, so conversion to 15hp might not be possible. Good to hear 9.9 vs 15 might not matter. I usually refer to Sea-Way Marine's website for p/n info but will try BRP's going forward. Am hoping NC wind lays down soon so I can try some speed tests with this 9.9.
 
Dave Clites said:
Thanks to all for the input. I bought a 1986 Johnson 9.9 over past weekend. It runs ok but may have some screws rusted in Midsection that could prevent access to Inner Exhaust Housing, so conversion to 15hp might not be possible. Good to hear 9.9 vs 15 might not matter. I usually refer to Sea-Way Marine's website for p/n info but will try BRP's going forward. Am hoping NC wind lays down soon so I can try some speed tests with this 9.9.
Click to expand...

Dave,

To the best of my knowledge, the Higbee hull is a planning hull and a 9.9 should get it up on plane. I agree, once on plane, one probably would expect negligible top speed difference between a 9.9 and a 15 hp engine. Where one will noticed the 50% increase in hp, is load capacity, in regards to getting that load up on plane. A 9.9 may jump the boat up with one man a dog and a few decoys. Add another hunter or a larger hunter, a few more decoys, another gun, the 9.9 will take longer to get that load on plane. This is where one should expect to see an actual difference with using a 15hp.

You can try swapping the jet in the carb but I doubt you will notice any difference at all except your gas consumption will increase a bit. I tried this back in my younger years and money was tight. (this was when the motors in question where current year models) Good luck in your adventure.
 
I've converted a gamfisher 9.9 to 15 by changing the carb and adding a tuned extension on the exhaust
by removing the powerhead, pretty easy to do and not expensive. Got the parts from a discount marine parts store before the interweb and don't remember the name. No noticeable difference in speed/power from seat of pants but did make it noticeably louder. There's a fin made for smaller outboards that I'm very impressed with It's performance , but can't myself come to put one on.
 
Bill, thanks for input/idea. Those Gamefisher outboards seem to be priced quite a bit lower than other brands. I'm not sure why that is--possibly parts availability??? I owned one a couple years back, it was a 2.5 or 3.5 hp, I put it on a 14ft canoe and it was scary fast. Dangerous, really, because it had no neutral. Pull the rope and be ready to hang on!

When I get my rig running (current step is getting my Sneakbox back on the trailer with a Transom Saver for supporting the 9.9Johnson) I will be testing speed with GPS so that I "know numbers" instead of intuitive feel. However even my "uncalibrated eyeball" can see the difference between not planing and planing!
 

Here's a link to a guy who made a "full conversion" of a Johnson 9.9 to 15. In addition to the carb and exhaust housing, he also mentions something about the leaf valves:

https://www.outboardguy.com/evinrudejohnson-99hp-vs-15hp.html

I'm really curious about the comments that a 15 won't gain you much speed once on plane. Generally, more hp doesn't gain much speed when in displacement mode, but once on plane, the boat's speed can be easily increased by adding power. Usually the bigger motor can either run the prop at a faster speed or simply turn a prop with an increased pitch. I have never heard of a planing boat that didn't run a great deal faster with 50% more power. That doesn't make sense to me.

Edit to add: I looks like clicking on the link doesn't work, at least for me, but if I copy and paste it into my browser it works fine. Not sure how these things work.
 
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Gordon Bartlett said:
I'm really curious about the comments that a 15 won't gain you much speed once on plane. Generally, more hp doesn't gain much speed when in displacement mode, but once on plane, the boat's speed can be easily increased by adding power. Usually the bigger motor can either run the prop at a faster speed or simply turn a prop with an increased pitch. I have never heard of a planing boat that didn't run a great deal faster with 50% more power. That doesn't make sense to me.
Click to expand...

I think the key to these comments is the application. If one was looking to prop out a race boat, then yes one would be reducing weight and upping the pitch. In a duck boat application, increasing the hp is more a function of being able to carry a heavier load with the same pitch. Most of us (not everyone) tend to take as much gear as the volume of our boats will allow. We keep the prop pitch geared for load capacity rather than pure speed.

I usually find myself cruising along at just above planning speed rather than flat out full throttle. Full throttle is reserved for getting on plane. In my situation, I've got too many river snags, sand bars and narrow channels to navigate that won't allow me to run top speed anyhow.
 
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Gordon Bartlett said:
In addition to the carb and exhaust housing, he also mentions something about the leaf valves:
Click to expand...

Some engines will use either a different leaf valve and /or a different stop plate and/or a different shim between the stop plate and the reed valve.

I checked the parts numbers for Dave's engine (1986 Johnson RCD) and the part numbers are the same for both 9.9 and 15 hp models.

Guys have been know to modify racing engines (dirt bikes, go karts, etc) by adding their own shims to increase the range that the reed valves can open. Doing this custom modification can be risky. The reeds have been know the crack and break off due to flexing beyond their designed range. A broken reed valve in the crankcase is not good. [;)] not saying I've ever done this or other risky and sketchy things to engines and vehicles.
 
Hi All, it has been many months since I updated on this subject. I bought (and plan to resell some of) a couple small outboards toward making my rig fully ready for this coming season. "Short version" is that with only me plus minimal fishing gear a 1998 8hp 2 stroke Evinrude is plenty of engine for this boat. In fact, at full throttle the boat gets a bit "squirrelly". I took a cruise with me and wife (total weight = 170 +130 = 300 lbs) and the 8hp was plenty of power to plane. I also acquired a 1986 9.9 2 stroke Johnson and it seems like more power than necessary and extra weight kills my back lifting on/off transom, flushing and storing etc. I still need to do some GPS speed tests on the 8hp, 9.9 hp, a mid-80's 6hp and a 1969 6hp which I discovered may not have been running optimally at time of last speed test--I have since installed a new impeller and thermostat on that 6hp.

Meanwhile, this Hurricane Dorian is going to keep me busy for the next week or so here in coastal NC.
 
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