Calling all cobra 7.5 owners

boristhebold · 04 May 2014, 06:29

Im going to look at getting more top speed by changing my prop as rpm doesnt reach max at the momemt.

Tomthat end im interested in whatnprops are fitted to other cobra 7.5 out there.

So what engine do you have what prop fitted make model diameter and pitch, what rpm hitting at what max speed.

Will compile the data and share.

Boris

Oscarguitar · 04 May 2014, 11:21

Mark,

What have you got on there at the moment? If you want to try my Rev 4 19 some time you let me know. Only take a moment to swap if the hub will fit and the thrust washer is the same. Can swap on the water (I've done it before, just be careful not to drop any thing) if you want to meet up some time.

boristhebold · 06 May 2014, 07:03

Thanks Oscar, I have a 14inch diameter with 19inch pitch, am getting about 5100 rpm and 43knots max when seas are smooth. Have tried a 14inch diameter 21inch pitch in similar conditions and while exceleration improved the top speed went down slightly to about 41 knots.

Having spent some time yesterday under my boat sanding a couple of minor gel coat repairs I had a decent look over the entire hull and I think im going to do some work on the hull to get it really niceley polished before I start trying other props. While parts of my hull the most visiable parts look ok on the surface when I look at the really bottom parts I doubt its been polished for X years so im going to sort it out and then do some trials with a 19inch pitch and a 21inch pitch again and take it from there.

I had a good look round the dry stack also yesterday and noticed a vast range of different props even on similar boats.

It seems to me prop choice is an art in itself. And based upon the fact that lots of boat owners have spare and various different props sitting in their garages I expect there isnt much work down on setting boats up in the first place with the ideal prop.

Wish there was some company out there who would buy up all the spare and unused props people have and then hire them out for testing for a a small fee.

Searider · 06 May 2014, 09:55

What engine have you got - it's not in your sig.

If you're not getting max rpm with a 19" prop, a 21" will be worse - getting you even less RPM.

How long has 5100rpm been your maximum - what I'm getting at could there possibly be a fault with your engine that's lowered your max from what it once was?
It seem a little strange that acceleration was better - normally it gets worse with a longer pitch. It's a bit like saying your car accelerates better in 4th gear than 3rd gear.

Oscarguitar · 06 May 2014, 09:57

Mark,

I have spent a few weeks under my hull getting it polished. Not fun but worth the effort cosmetically, I'm not sure it made much difference to the speed, however it does eliminate one possible problem. Thomas at propellor revolutions used to race ribs and commented that the Cobra hull doesn't do much over 45 knots with out a huge amount of power. Mine tops out at 43 knots just like yours. The ones with twin 250s are much quicker but so they should be! So you could be chasing some top end that just can't be had with your set up.

Props are an art form. Not only pitch and diameter but progressive rake, the shape of the barrel and many other factors will have a difference. The Rev 4 for instance is a "stern lifting" prop. I can plane at 10 knots, and the boat just pops onto the plane with no bow lift. The rev 4 can also be run very high without ventilation, and raising your engine will reduce drag. I have tried a 3 blade (can't remember which) but without the stern lift I couldn't plane below 15 knots! However the downside of the Rev 4 is that it is very difficult to get the bow up when I want to (following sea) and I reckon that I could get more top end if I could reduce the wetted area once up and running.

The best source of information on props is the Propgods forum. Ken is based in the states and does pretty much what you suggest, he has a huge supply of props and will send them out and let you try them for $35 but not much help to us over here. He will make suggestions if you ask the question on his site.

Before you change props check that your engine height is correct, the AV plate should be just visible above the water when at a decent cruise speed. Many manufactures set the engine too low (mine was so I raised it as much as I could it still is one hole too low but raising it further would mean re drilling the transom which I'm not prepared to do) and you could gain 1-200 rpm just by doing that. RPM of 5100 does seem low so you may actually need to go down to a 17 pitch to allow your engine to reach the top of its rev band to access max power. If you can borrow a 17 from someone that should give you an idea at least.

Give me a shout next time you are out, I am a self confessed prop geek and would like to help is possible.

Oscarguitar · 06 May 2014, 10:00

Have a look at this article on engine height. Not Verado specific

http://www.veradoclub.com/smf/index.php?topic=137.0

Searider · 06 May 2014, 15:45

Good article that.

boristhebold · 06 May 2014, 15:51

Engine is a 200HP 2 stroke HPDI.

I rechecked carefully my prop, its not 14inches its 13 and 3/4 inches.

To be honest 43 knots is enouth. I think im going to clean and posih the hull and try a 14inch with 17inch pitch. According to the vast articles ive read recently the 2 inches smaller pitch may hopefully find my missing 400rpm. Will let you all know.

boristhebold · 07 May 2014, 14:35

Finally found what should be the correct prop for my engine, after hunting long and hard found something which listed exactly my engine model number as in Z200Neto rather than all the other 200HP Yam HPDI models, there are about 10 models.

Listed as being a prop with 14 and 1/4 inch diamter and 17 inch pitch as apposed to a 13 and 3/8 diamter and 19inch pitch on at the moment. So that should hopefully get me the 400 RPM back.

Will post details of the result when ive tested hopefully next week.

boristhebold · 19 May 2014, 09:05

Boat now back in the water, tested with new prop fitted a 14 and 1/4 inch diameter by 17 inch pitch.

Yam 2 stroke 200HPDI full throttle rev range 4500 to 5500

On old prop which was a 13 and 3/4 inch by 19inch pitch getting around 51k rpm at 41, 42, 43 knots.

On new prop I am now easily getting 5500 rpms (briefly saw 5600 and backed off slightly) at 41 knots but sea conditions not ideally flat, so i expect I would get more speed on calmer water.

One thing I did notice was that the engine seemed to run quieter and smoother, I seemed to be able to cruise along at a given cruising speed of say about 27 knots with little or no adjustments needed. dont know if im imagining that but does seem smoother. Any pointers as to why ?

My hull still needs a good clean up and polish.

Oscarguitar · 19 May 2014, 09:12

Your old prop might have been out of balance. I have had problems with a couple of props one of which was brand new. Also it could be that you are in the right "gear" now using the 17" prop so the engine is not lugging.

Finally, have you checked the height of the anti cav plate at cruise? If the engine is too low you could find that by raising it up a hole or two you could run the 19 and get a couple of hundred more rpms.

boristhebold · 19 May 2014, 10:37

Quote:

Originally Posted by Oscarguitar

Your old prop might have been out of balance. I have had problems with a couple of props one of which was brand new. Also it could be that you are in the right "gear" now using the 17" prop so the engine is not lugging.

Finally, have you checked the height of the anti cav plate at cruise? If the engine is too low you could find that by raising it up a hole or two you could run the 19 and get a couple of hundred more rpms.

Thats my next task, im going to mount my drfit action video cam on the boarding ladder and film whats happening in terms of water flow at various speeds and trim for viewing and analysis later.