15 hp 4 stroke

[sjohng]

my boat is a alum 14' deep v tiller weighing about 800 lbs with me and gear.
current prop is 9" x 9p ,motor is 15 hp 4 stroke 2007 mercury long shaft rated @ 5500 wot rpm (note on motor)
installed a tach at wot i'm at 5900 rpm and 20.5 mph
any advice on what prop I should be using ,before I blow up my engine.
thanks steve

 

[craze1cars]

Re: 15 hp 4 stroke

Sounds like you need an 11 pitch to me.

 

[sjohng]

Re: 15 hp 4 stroke

what will that do to my speed and ability to get up on plane

 

[steelespike]

Re: 15 hp 4 stroke

Your motor probably developes its hp at 5500.
According to present day specs wot rpm range is 5000-6000 rpm.
For strong hole shot and ability to handle a load you need to be propped close to your max rating.
Going to a 11" pitch usually will lower rpm about 400.It will slow hole shot and will fade faster as you add weight.
When lightly loaded it may be a little faster.I think if you must change the prop that a 10" would help to maintain power for hole shot etc.
If your boat is truly a Deep V it will have about a 20 degree dead rise at the transom.
A true deep V will generally have a slower hole shot and slightly slower top end but will usually ride smoother than that of a boat with a relatively flat run aft.

 

[sjohng]

Re: 15 hp 4 stroke

my concern is that over revving the motor will damage it.

 

[drewactual]

Re: 15 hp 4 stroke

over-revving, while bad, isn't as damaging as lugging, by a far cry...

you may be a candidate for a vented prop... I've made like four posts here, including this one, and each of them spoke of using vented props... I reckon I'm more of a fan of that than I realized?

it's been my observation, for what it's worth, that small boats are more about the differences in load on the boat, while larger boats are more about the boat... I'd guess it a game with percentages of weight- a smaller boat like yours, with one extra passenger, raised it's displacement in percentages much more than a larger boat would have with the same passenger would have.. it pays to have a versatile prop in that circumstance, or two props to use depending on what you're doing..

If I were you, I'd try a 11p vented prop, or a 10p non-vented, but slightly larger in diameter. moving to a four blade, I'd use a 10p vented or not...

If you choose to use a 10p four blade vented, (which I think will suit your needs) make sure and keep similar diameter, and have one with an aggressive 'progressive pitch', which would make the 'average' pitch 10p, but the leading edge around 8.5p and the trailing edge around 11.5p, and with a decent cup. This will allow you to rev faster (vents), punch the bow out of the hole (water off leading edge- shallower pitch of the blade), and lift the stern as it bites in (water forcing off of trailing edge of higher pitch).. the cup will allow better bite in turns.

 

[steelespike]

Re: 15 hp 4 stroke

Based on my research your aren't over revving at 5900. Ask a couple of Merc dealers what the rpm range of a 15 4 stroke is.
If you run light most of the time go with a 10" I don't think you will like the performance of an 11" prop.
Of course you can throttle back a little for cruising speed and save rpm and fuel.
If you increase pitch the motor may actually be working harder. Taking longer to plane and struggling more with a load.Probably won't cause any damage but may use more fuel in certain situations may not be up to your liking.
Venting a prop can be tricky as suggested in the above post. But if you don't damage the prop you can plug the holes if they don't work right. There are plugs for vented props, don't know if they are just generic plugs you insert in the holes or are special.

 

[sjohng]

Re: 15 hp 4 stroke

I can't find a manual that covers 2007 they all stop at 2005 and start at 2008
my serial # 1B468399 which is listed as a 2006 but stamped on the motors label is 2007 along with 5500 max rpm
2005 wot is 4500-5500... 2008 wot is 5400-6100
maybe I worry to much

 

[Silvertip]

Re: 15 hp 4 stroke

The reason boats have a throttle lever is to adjust engine speed. You don't need to run wide open all the time. Throttle back when you get on plane. If 5500 is the recommendation then a 10P would be appropriate. If 6000 is the recommendation, keep what you have. And should you desire to change props, remember this: Identical props from two different manufacturers may not and generally do not, perform identically. Hence, going to a 10P may or may not have the desired effect. Prop selection is a "crap shoot" especially when working with multiple manufacturers.

 

[drewactual]

Re: 15 hp 4 stroke

Silvertip nailed it... It would be beneficial, imHo, if blade geometry were more transparent... That progressive ramp I spoke of comes into massive play... The surface area of the blades, the effective surface areas of the blades, the angle of attack of the.cutting edge.. the height of cup from the base pitch and the distance it takes to get there, all of those seemingly miniature differences add up to a prop with advertised same specs from different makers and/or even different product lines behave totally different... This isn't even interjecting the different demands od engine and hull, weight in displacement and distribution, trim and mounting height, average seas/waterways... Proper propping is more akin to black magic than science.

 

[craze1cars]

Re: 15 hp 4 stroke

Easy answer to your RPM range question...contact Mercury. Their website has a tech service phone number. Give them the serial # of your motor and they'll tell you the rpm range, right from the horse's mouth.

I'm 99% certain your recommended operating range for the 2007 is 4500 to 5500. And the very fact that "max 5500" is stamped right there on a sticker on the side of the motor is pretty much a slap upside the head telling you what the max is. Thus my 99% certainty.

I'm also quite certain that occasional blasts to 5900 will be harmless. But if you run full throttle all the time, just pitch up a little bit to slow it down.

 

[sjohng]

Re: 15 hp 4 stroke

thanks for all the information, I think I'll step up to a 10 pitch , you guys have been very helpful

 

[steelespike]

Re: 15 hp 4 stroke

Good luck and please let us know how you make out.