Prop/RPM Questions Nissan 15HP 2 stroke

[Bronlonius]

I now have a working tach and correct motor height, so I'd like to get the optimum prop. (Thanks Sea Rider for the recommendation on a hardline tach, it works great).

Here's what I have right now:

Boat: 12' tracker V12 lite. Has a casting deck in the front, battery and 2 3gal gas tanks in the front.

Outboard: 1996 Nissan NS9.9D converted to 15HP. Outboard is on a jack plate, and dialed in at about 1" above the keel.

11 pitch stainless prop:
with 1 person: 5100 RPM @ WOT, roughly 20mph (GPS reading is hard to get on my lake for some reason, but it's generally been between 19mph and 20.5 when it works).

The boat has great performance with one person, but takes a while to plane with 2. I haven't gotten a tach reading with 2 people in the boat yet, but it's quite obvious that the prop has too much pitch for the extra weight of another person.

What I'm hoping for is to get a prop that brings the WOT RPMs to the max (5500) with one person, and is still able to have enough torque to plane the boat with 2 people. So assuming 200 RPM per inch of pitch, I think a 9 pitch prop would be ideal.

Questions:
1. Stainless vs. Aluminum - Am I correct that a stainless prop is essentially the equivalent of one pitch above if I get an aluminum prop? (i.e. a 9 pitch stainless is going to give the similar WOT RPM as a 10 pitch aluminum). Is stainless worth it on such a small outboard?

2. Am I better off getting 2 different props(Keep the 11 pitch stainless one for just me, and get one for 2 people/loaded) or am I going to be able to get away with having one that is a happy medium?

3. If I do end up with 2 different props.... are there any props that have a swappable hub system that would make changing props for different loads a quick and easy thing? (Yeah, I know it's just a cotter pin and a nut, but it always seems to take forever to get that cotter pin back in there.

 

[steelespike]

Re: Prop/RPM Questions Nissan 15HP 2 stroke

Actually a ss prop usually has some cup and a cupped prop calculates like an inch bigger al prop ,not necessarily rpm but speed.
I do think you will get better responce with a 9" pitch.and it may improve your slip which appears high now at about 21%
probably because the pitch is too high.A 4 blade will further improve your hole shot and may also lower your slip.
Changing material make the results more unpredictable.
Actually if you had a big full sized bare 14 footer it might get up easier and run faster with the 11" ss prop. Its easier to plane with
the bigger footprint.

 

[Bronlonius]

Re: Prop/RPM Questions Nissan 15HP 2 stroke

Actually a ss prop usually has some cup and a cupped prop calculates like an inch bigger al prop ,not necessarily rpm but speed.
I do think you will get better responce with a 9" pitch.and it may improve your slip which appears high now at about 21%
probably because the pitch is too high.A 4 blade will further improve your hole shot and may also lower your slip.
Changing material make the results more unpredictable.
Actually if you had a big full sized bare 14 footer it might get up easier and run faster with the 11" ss prop. Its easier to plane with
the bigger footprint.

Good info, thanks. The price of a stainless prop is about double what the aluminum is, but you make a good point.... If I buy the wrong aluminum prop, it will end up costing me the same as getting the right stainless one. I do actually have an 8 pitch aluminum spare, but it's pretty beat up so I don't know how accurate the results are going to be with it. Now that I have an accurate tach reading I will probably do a test run with it though just to see what it does.

So one more question, what's a "normal" slip percentage?

 

[steelespike]

Re: Prop/RPM Questions Nissan 15HP 2 stroke

Typical slip is about 8 to 12%. on the typical planing boat . Larger/heavier/hard to push boats may be higher.
Usually as rpm goes down slip will go up. Your little 145 lb boat may be struggling a little with the weight. I'm guessing 600 lbs boat,motor,you gas tanks,battery gear
The deck,battery and gas tanks up front probably help it to plane but may be holding the bow down.Is there a trolling motor involved with the battery?

 

[Bronlonius]

Re: Prop/RPM Questions Nissan 15HP 2 stroke

Yes, there is a minn kota 30lb thrust trolling motor(transom mount), it runs from the same battery that everything else does. Also, there is an electric anchor mounted on the front of the boat.

I'm thinking about removing the casting deck and either rebuilding it with lighter materials, or maybe just not having it... the previous owner made it out of 3/4" OSB and 2x2s, he completely overbuilt it, I'm guessing it weighs about 75lbs, maybe more. I don't really use it for standing on, it really just hides the gas tanks and battery and that's about it.

 

[Sea Rider]

Re: Prop/RPM Questions Nissan 15HP 2 stroke

Hi Bronlonius,

Nice to read the Hardline tach works well, more simple and less cable to install than TT. You could install on front cowl, between kill cord button and fuel connector, fits perfectly there assuming you are tiller steering your boat. Don't know if going for a SS prop will have any benefit, those are usefull on more powerful 50 + engines not portable ones. Better buy a Solas alum prop.

Go for one that deliveres near 5,800 rpm stated for that engine at wot with you alone and rigged as usual. With added boater max revs will decrease accordingly probably 200/300 less but in between min 5,300 max 5,800 rpm safe working parameters.

Some issues : Your engine has been upgraded to a 15. You have swapped throttle 9.9 cam for the 15 HP, right ? Do you have a full opened 180? butterfly at full throttle, check, if not, you're not achieving full 15 HP. Will check 15 HP timing advance latter.

Which rpm was your current factory delivered prop achieving with you alone at wot, assuming it's a 8.5 pitch prop. Should test with 2 boaters also to see how many rpm engine is decreasing at wot. What's the max HP engine your boat is rated for ?

Happy Boating

 

[Bronlonius]

Re: Prop/RPM Questions Nissan 15HP 2 stroke

Is the max rpm 5500 or 5800? The operators manual I have says 4500-5300 for 9.9hp, and 4750-5500 for 15/18hp. This is an NS9.9D, so 1996 vintage, do the newer models rev slightly higher?

And yes, the throttle opens to 180, and the farthest timing mark lines up with the split in the casting at wot, which I believe indicates it is advanced to 25 degrees.

I do have a beat up 8 pitch aluminum that I got for a spare, now that the tach is working I'll give that one a try and see what I get.

 

[Sea Rider]

Re: Prop/RPM Questions Nissan 15HP 2 stroke

Yes newer 15D2 revs 5,200-5,800 at wot, the 9.9 runs less due to carb is not throttling farther because of cam shape, so less max rpm is achieved. Being both 9.9 & 15 HP 250 CC is not an issue for the 9.9 to top max rpm of the 15. So you have a SS 11 & 8 alum pitch prop, right ? What's the max HP engine your boat is rated for ?

Happy Boating

 

[steelespike]

Re: Prop/RPM Questions Nissan 15HP 2 stroke

I think the V12 lite is rated for 10 hp.

 

[Bronlonius]

Re: Prop/RPM Questions Nissan 15HP 2 stroke

Correct, it's rated for 10HP. I realize this is technically overhorsed, but since the 9.9 and 15 are the same weight, I figure the only difference is the force on the transom from the extra HP. The transom has been significantly beefed up with the jack plate(jackplate is made from 1/4" aluminum angle, and there is a 1/4" aluminum plate on the inside of the transom). I bought the motor thinking it was still a 9.9, but after buying the 15hp cam discovered that someone had already converted it to 15HP.

 

[Sea Rider]

Re: Prop/RPM Questions Nissan 15HP 2 stroke

So you have engine/transom height opt seated, engine well trimmed, well ballanced boat, right ? If so, A 3.65 Mt alum boat with a healthy running 15 HP should perform well. Seems is just a matter to dial best prop pitch so engine revs higher. Most portable engines factory delivered with standard props, usually achieves near full max rpm on light boats, and that's lightly loaded and solo driving.

Factory delivered 8.5 standard pitch should perform well, a 8 pitch probably will be over reving, make test trials with you alone and then with extra boater and check max rpm achieved on both outings tested prefferibly on flat calm no wind waters. Could you post some pics of boat/engine combo & back transom area to have a look at the engine's jack plate ?

Happy Boating

 

[Bronlonius]

Re: Prop/RPM Questions Nissan 15HP 2 stroke

Here are a few pics of the jackplate. The one of the whole boat on the trailer is from a few months ago so not the perfect motor height yet. The one that shows the prop is the current height(thats obviously the aluminum 8 pitch). And yes, the motor is a 20" shaft, so the jack plate is way up there since the boat has a 15" transom. The jackplate is set to 8.5" above the top of the transom, with 5.5" of setback. The bumpers shoved between the transom and jackplate help keep the splash for coming over the transom when backtrolling or slowing down from WOT, and also add some buoyancy to help compensate for the motor being farther back.

I also forgot that there are still doel fins on the motor, I haven't tried running without them yet, but that might have something to do with the high slip, or maybe I should try a different GPS... the one I've been using(iphone) takes almost a minute to get a reading and doesn't read at all under about 15mph so I don't know how accurate it actually is.

I am hoping to go fishing tomorrow or saturday, so I'll try without the doel fin and probably switch to the 8 pitch to get some more readings and see what it looks like. I'll also try and snap a picture of how it sits in the water. It seems very well balanced to me, but maybe I have too much weight up front.

 

[Bronlonius]

Re: Prop/RPM Questions Nissan 15HP 2 stroke

Okay, I managed to get out today and get some test results with the 8 pitch aluminum prop, and brought my ipad with, which has a much better gps than my older iphone. I was actually quite surprised, surprised enough that I might run the 11 pitch once more just to see what the actual speed it gets is.

Anyways,

8 pitch prop, with just me and my gear in the boat:
21.6mph @ 5310 rpm

8 pitch prop with me, my gear, my wife and my 3 year old:
19.3mph @ 5130 rpm

With just me, the boat is up on plane almost instantly, and gets to the max rpm very quickly. With the family, it takes a while to get the bow to go down and get to max rpm, but it wasn't unusable like it was with the 11 pitch.

I was expecting the rpms to be higher when going with the 8 pitch aluminum, and also expecting a bigger difference in top speed/max rpm with my family in the boat.

Anyways, I still need to test with the doel fins removed, and after that I'm going to test with the casting deck removed to see if either will bring the rpm/speed up a bit.

 

[Bronlonius]

Re: Prop/RPM Questions Nissan 15HP 2 stroke

Well I did another test today with the doel fins off. With just me in the boat, and the 8 pitch aluminum prop, I got up to 5500rpm, but the bow really goes up when coming out of the hole, and it porpoises pretty badly, even when trimmed all the way in. The fins are definitley causing some drag because the boat accelerates better withoutout them and gets another 200 rpm at wot, but unfortunately I think I need to have them to stop the porpoising.

 

[Bronlonius]

Re: Prop/RPM Questions Nissan 15HP 2 stroke

Heres a good picture of how the boat sits in the water. Obviously the back goes down a little more with me in it, but not much. The back seat also has a slider under it so I can slide it forward a few inches to move my weight towards the middle.

 

[steelespike]

Re: Prop/RPM Questions Nissan 15HP 2 stroke

You could buy or make some wedges for the motor so the angle is better for tucking the prop closer to the boat.
Your lightly loaded rpm and speed aren't quite right as slip is less than 1%.Is the speed by gps?
hard to tell in the pictures but the transom doesn't seem to have much angle.
Is the antivent plate barely visible at speed?

 

[Bronlonius]

Re: Prop/RPM Questions Nissan 15HP 2 stroke

The current leg height is perfect, The antivent plate runs right under the surface of the water and I get a small amount of water gently splashing off the upper plate. I don't have any pictures of the splash, but Sea Rider has a thread in the Nissan/Tohatsu section with pictures of the effects of different leg heights, my upper plate splash looks identical the one that gave him the best results.

I was thinking about the transom angle, and after looking at pictures of other boats I agrees that looks like it's pretty straight. In the picture of the boat on the water, the trim is at the second closest hole to the transom, which works perfectly with the doel fins attached. There are 4 or 5 more holes for trimming out which would probably send the boat to the moon if I took off fast without the doel fins. I can probably rework the jack plate to get some more tuck, worst case I'd just have to buy some more aluminum for the side pieces and re-drill the slots at an angle.

I used the gps in my ipad (v3) to get the speed. Not sure how accurate that is... Either that or the tach is wrong, or the prop is not actually an 8 pitch?(It has the number 8 cast into one of the blades, no other markings that I can find). I was expecting a bigger difference in RPM than 200 between an 8 pitch aluminum and an 11 pitch stainless, but maybe the prop is actually an 8.5 or 9 pitch? It looks like it might have been repaired at one point, you can see welds that were ground down where the blades attach to the hub.

I did notice that while the boat porpoises/lifts badly without the doel fins, that it feels much more "natural" on turns without the fins. I think that when turning the doel fins dip in deep and create some weird forces back there, probably not noticeable in a big console boat but I can definitely feel it with the tiller. Hopefully getting more tuck will be enough to stabilize the boat more. Do you think more tuck is going to add more stress on the transom? It seems like it would want to "twist" it more. I wonder if it would be worth making a knee brace that attached the transom to the center rib, or if that's overkill. The current setup doesn't flex at all.

 

[Bronlonius]

Re: Prop/RPM Questions Nissan 15HP 2 stroke

duplicate post

 

[Bronlonius]

Re: Prop/RPM Questions Nissan 15HP 2 stroke

Well I bought an 8 pitch solas amita from iboats. Free ground shipping, but I still got it the next day (shipped from a warehouse 1 state away). Anyways, with just me, and almost out of gas, it gets up 5600rpm. Couldn't get a gps reading but it feels pretty fast. I think the other 8 pitch aluminum was either reworked to a 9 pitch at some point, or is just too beat up to give max possible performance. But based on the speed I got with the old one, I think it's probably not actually an 8.

Anyways, will try again with both tanks full, and with the family and see what kind of results I get. I think I'm pretty close.

 

[steelespike]

Re: Prop/RPM Questions Nissan 15HP 2 stroke

I think the 8 will give plenty of power and you'll think it's different motor with a load.
I think a 9" may be faster with a light load at about 5400 rpm but may not hold up as well with a load.