could it be propped wrong?

[jere1972]

Re: could it be propped wrong?

there are bottom through hull water intakes but they are about 24" away from the center line, bottom is relitivly clean, stained but clean

 

[QC]

Re: could it be propped wrong?

Although those intakes could affect it, I still think this is primarily about the wheel

 

[jere1972]

Re: could it be propped wrong?

could my trim tabs be a problem, when we remounted them after rebuilding the transom, we located them using the same screw holes, when the tabs are in there full upright/in position they are parallel with bottom, not any deg. up or down but parallel, and are a 1/4" from the bottom edge and 3" from the sides, I just realized I had an earlier thread regarding prop pitch (sorry for adding another on same topic), but I was looking at what that was about, appears when running a 23" mirage, hitting 4200 at WOT. and 31mph on calm water, I am a bit lost as to what I may need to go look for prop wise, there seems to be many factors in prop selection that I was never aware of, I know if you walk into west marine they just have a prop in a box, can aluminium be used, they are much cheaper,

 

[QC]

Re: could it be propped wrong?

I think Aluminum could be fine for you. Seems like you should look for big round "ears" and largest diameter you can swing. Your tabs are not optimum as they should be up a few degrees when not deployed. Based on that, and your slip numbers, I would try shifting some weight aft. Can you trim up at all?

 

[tazrig]

Re: could it be propped wrong?

Have you thought about a 4 bladed prop? That would certainly give you more bite and less slippage.

 

[jere1972]

Re: could it be propped wrong?

Yes i can trim up, I read up on 4 blade props last night, they seem to better your low to mid range, giving better hole shot, which I don't think i have a problem with now, but they effect top end performance, this was just what I read, I also saw a posting about getting a prop tweaked, more or less was in regard to the mirage, where they said the mirage+ was a mirage with more cup, and mercury marine made the + because so many people were taking their new mirage to a shop for more cupping. I'm wondering if thats what I need to do instead of creating a stock pile of props, taking my prop or props to a place like Admiral Propeller explain what I have going on, and what I like to achieve, they should be able to tweak my current prop??????? and I'm guessing here on this. I ran my numbers through a slip formula, I'm guessing I'm around 37%, where I should be around 20-25 based on boat type, and hull drag may be my greatest problem to over come, it may just be a poorly designed hull,it is a Renkin, not enough bow area coming out of water, thats where I thought my trim tabs could be a problem. I never really thought there would be so many factors, buy hull, fix hull, install motor,done was my mindset, guess
I was way off.

 

[emilsr]

Re: could it be propped wrong?

Before you start playing with props, verify the accuracy of your tach. Use a shop tach and see if you are indeed turning 4600rpm. If the tach in your dash is reading on the high side that would explain a lot.

 

[QC]

Re: could it be propped wrong?

TOTALLY agree with emilsr. Tach verification first. $19 on eBay for a cheap "Tiny-Tach" knock off.

not enough bow area coming out of water, thats where I thought my trim tabs could be a problem. I never really thought there would be so many factors, buy hull, fix hull, install motor,done was my mindset, guess
I was way off.

LOL, yep, waaaaay off

This is why I asked about up trim. You can achieve a lot of improvement with proper weight shift and trim. I am concerned about your tab positioning as they may be limiting your ability to get the bow up. Generally bow goes up, speed rises and slip decreases. HOWEVER, even with tabs possibly limiting you, if the bow is too heavy it won't matter and up trim will lead to "blow out" which is the prop grabbing air off the stern and basically free wheeling. So if you can make her blow out before she starts to "porpoise" (rhythmic rise and fall of the bow), then I would suggest shifting weight from the bow to stern in any way you can. I would also consider remounting the trim tab rams, but that's a major biotch as the hydraulics run through a hole all the way through the transom . . . Doh! (Bennett's, Lenco's would be easier to remount I think.)

Yes your prop can be improved, but I think even with work this is probably the wrong prop to start with. The fact is you NEED two props anyway. One as a spare. So I would stop spending any time, or money, on the Mirage. It is now your spare. Talk to as many good prop guys locally as you can, and get something better matched. I still think three blade, and bigger, rounder ears. Aluminum is very capable for this. Oh, and I would finance this, and maybe end up in the black, by selling the 23" one you have. Selling props on eBay or Craigs list is easy.

 

[Tail_Gunner]

Re: could it be propped wrong?

I have a 95' Renkin seamaster 2688, the boat originally had a 7.4L Yamaha hydradrive, I repower the boat with a 91 7.4L merc/Bravo 1 1.50 that i pulled from a damaged 23' Cuddy, the guy told me the boat used to run 60mph before damaged,
the merc came with a 23" pitch mirage not sure on the dia, installed in my Renkin, the boat seemed to have difficulties getting to a planning state. with a top speed on relative calm seas 1' or less @ 28mph, I purchased a used but never used ss Mirage 19", she gets up on a plane great , top speed seems to be around 33-34 (GPS), on a calm day seas @ 1' or less, running the 23 my rpms would barely hit 4k maybe trimmed out, with the 19 I can hit 4600 trimmed out with no problem, I normally run around 3500 about 25-26 mph, I never had a boat this heavy or long, so not certain if I'm running where I should be, the yamaha had a 19", but it feels to me I should have more top end, with the 19 it makes no difference if I have 2 people on board or 6 people, she runs out the same, the hole shot now seems good but I'm thinking my top end is lagging. but I'm comparing things as to how my 20'ran with a johnson 225, which would top out around 56 running a 17"

Here lets play some number's over. Just a thought that boat will never reach 50 with twin drive's its a old 7.4 maybe 300hp fresh in the box
19p..1.50....4600 nets 55 actual 28 55% slip
18p nets 52
17 nets 49
16 nets 46 now slip the prop to 15% and 40 mph.....26' boat 300hp 40 mph that's moving right along.

Since a lot of work has been done to the boat you may want to check your timing but the way that motor pushes a 19 to 4600 it seems healthy....If your tach is correct.

but I'm comparing things as to how my 20'ran with a johnson 225, which would top out around 56 running a 17"

A complete differnt planet...trying running that 20' boat in 4' wave's....now the 26 just speed bump's..

That's a lot of boat for a 40 mph and a single 7.4

http://www.certifiedsales.com/listings/YS070163.html

 

[emilsr]

Re: could it be propped wrong?

What does she weigh?

 

[Tail_Gunner]

Re: could it be propped wrong?

What does she weigh?

That would be a 5000lb plus boat.

 

[jere1972]

Re: could it be propped wrong?

boat weight I really don't know, I read something the hull weight was around 5600, I don't know if that was with engine, this Particular hull came with many power options , outboard or sterndrive,and I've seen them online installed with factory 5.7's, as far as weight shift, not sure what can be shifted no one ride in the cabin or up front , the trim tabs hose runs external I'm guessing it should be no problem to shift rams higher I would just have screw holes to patch in my new transom, I'm concluding and agreeing with everyone about the mirage now( lesson learned, ask someone who knows be for buying something you know nothing about)., and taking it somewhere would be time and money wasted, since mods would cost more then a new Aluminium, and they can't add surface area, I looked online props that are larger dia seem to be less pitch, 16x16, assuming I can swing a 16", I'm a bit confused with Tail gunners post, seems then dropping pitch would give me less top end but less slip???is that what I need to consider, I'm guessing I will never see 50+ from this setup, I'm hoping she should run some where 40+, she will do 33-34 now. I certainly do not want to sacrifice my cruise area of 3000-3500, top end would seldom be used, I'm just thinking she could be more efficient through out all her ranges, for all I know 3500 cruise rpm may be way to high, she will fall back and plow @ around 2900-3000, I just figured 454 WOW what a beast. and expect more, I know she is a torque monster. as to trim blow out, she certainly does that, and she never gets to that porpoise state before she blows out,how ever I have had it to a point where she did go a bit faster 36-37 maybe, just before blowing out along the intercoastal, however her RPMs would excede 4600, so I stop trimming when RPMs hit 4600 I would have thought there would have been a rev limiter in the ignition system but guess not, the tach should be correct it's new, the old tach worked and gave the same readings, it was sticking at times you had to thump it. think i may be struggling to find a good prop guy in the area, places that sell props, are just that they sell props with my other boat I had to tell them what i needed and they would go pull it from stock.

 

[Tail_Gunner]

Re: could it be propped wrong?

Here lets play some number's over. Just a thought that boat will never reach 50 with twin drive's its a old 7.4 maybe 300hp fresh in the box
19p..1.50....4600 nets 55 actual 28 55% slip
18p nets 52
17 nets 49
16 nets 46 now slip the prop to 15% and 40 mph.....26' boat 300hp 40 mph that's moving right along.

Those numbers are solid performance number's, how they were used...to determine what the prop could achieve with the boat taken out of the equation. It was simple enough to walk down the prop size until a speed was achieved that was somewhere close to reality..In this case 16p is where it made some sense..and it took a high slip figure to make that work out. Call it a Wild A@# Guess..WAG but with some reason.

Find a donor and try a 16 i think you will be surprised..yes you do have a tourqe monster and you will feel it...

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++


just before blowing out along the intercoastal, however her RPMs would excede 4600, so I stop trimming when RPMs hit 4600 I would have thought there would have been a rev limiter in the ignition system but guess not.

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

That should explain a lot. the hgh slip and the quality of the mirage there a great prop. The prop is just so large you have to trim it to the point where she slips badly and builds rpm. Could i be wrong oh yes but i do not think so. One last thing as you have stated at max trim your still stuck to 4600...the engine should spin right to 5000 like now with as much slip you have going on. Take the time to find out the exact distributor you have and time by the book. You would be surprised by how many make simple mistakes by using the wrong procedure...There is more than one..Do you have a tbolt 1V..V or a automotive type with vacum advance..Don"t laugh it happpens all the time. I am assuming this is a Carbed engine is it not??

 

[QC]

Re: could it be propped wrong?

New doesn't mean much for a tach. There is a setting in the back. Make sure it is right for a V8 although I do think the numbers are believable. Your over rev comment is interesting, and could indicate tach issues as well. We really shouldn't be evaluating anything unless you can truly run WOT trimmed out all of the way. Please verify the tach. It just eliminates any confusion.

Also, as Tailgunner noted, even though that is a "beast" and a 454, it is a conservatively rated example and only 300 hp at the prop. Many small blocks would outrun it today. Bulletproof setup, but not a rocket.

 

[jere1972]

Re: could it be propped wrong?

Ok kinda makes sense, but I'm still a bit confused, earlier post suggested getting a bigger prop for more bite, I can understand overtrimming to get to 4600, but isn't that the point of trimming, My timing is dead on factory spec 8deg btc @idle 600rpm she advances properly, and she is factory thunderbolt, I pretty confident the engine is tuned where she should be, so do I look for a larger dia lower pitch, or lower pitch lower dia, QC suggested strongly get big eared prop, I'm most likely gonna have to rule out SS, can someone give me a suggestion as to what size, manufacture, and model Alumin prop, I read the Bravo 1 can swing upto 16", is this correct,
I'm still little lacking as to what is causing the high slip, is it because the prop is just to much for the weight of the boat, and the Engines power is just spinning the prop and the weight and size is keeping her down, kinda like flooring the gas in your car and the tires break and spin, the car still moves slowly but the tires are smokin??????

 

[emilsr]

Re: could it be propped wrong?

You do have lots of torgue, and in fact that engine is set up to run 4600 MAX. If you dropped a couple hundred rpm that wouldn't hurt a thing.

Yes, you will probably have to drop in pitch if you go up in diameter, but if it gets prop slip down to 20% or less the net result will be better performance. I would think mid 40's speed under good (cool/light) conditions.

....But we're all just guessing 'till we're sure the tach is right. Mine reads 150rpm high at 5,000. Has since it was new. On the boats I've checked, that's been the norm. Don't go prop shopping just yet.

 

[jere1972]

Re: could it be propped wrong?

OK, seems like I need to do some further testing in regards to true RPM, I'm heading out next Saturday, what and where should I be observing my speed to Rpm, is it only @ WOT, or should I'd be checking at several points in my RPM range, I'm not even certain where I should be cruising @ with these engines, they are not Outboards so I know they are not suppose to run near WOT for long periods, I will verify again, but I believe @ 3k shes running 17-19 mph, but in calm water she would drop off and plow, on light-mod chop is where she held her plane @ 3K, in the 3.5-3.7k is where she is normal ran, and shes around 24-26mph depending on seas, again this is all what my mind remembers, so I bring some paper and a pencil and confirm and write it down, just need to know where and what I need to note. and what kind of test tach to I need?

 

[jere1972]

Re: could it be propped wrong?

Thanks, I was looking for props, credit card in hand, think I'll be holding off now.
Oh I forgot something, the gearing, I stated 1.50, thats only what i was told by the previous owner of the engine/drive, and he seemed to pause till I threw out 2 options, anyway to know from an external view????? the drive was completely sanded and painted no original decals left? or does it matter?

 

[Tail_Gunner]

Re: could it be propped wrong?

Timing both the 4&5 they are different. Done beating that horse

http://www.boatfix.com/merc/Techbk/95/95HGB4.PDF

Prop Diameter small large..Large diameter turn's faster...but the blade area is reshaped known as a big eared pusher..lots of blade area... works well on large heavy boat's. But you do have a nice deep v boat there and a 4 blade with a average diameter may work better..think lift and wetted surface. Gonna be a big differncene... no the pusher is the way to go i think...the smaller diameter has less drag and will be faster in some area's 35-40 hph area...Its just splitting hair's....Cant you find a cheap 16 used alum and give it run..if your slip changes and speed stay's the same ...that dog is hunting now...:lol:

 

[QC]

Re: could it be propped wrong?

Personally I still think bigger blade and stick with 17. He is over revving now, and like emilsr said, the 300 7.4 will be fine a little lower. (assumes tach is right...)

I am pretty sure you are safe with the gear ratio as that is all that was shipped with the 7.4 in 1991 according to my docs (they have been wrong before). The way to verify is pull the plugs, put her in gear. Bar the engine over say 10 time an d count how many times the prop turns. Divide 10 by result. 7.5 times equals 1.5 GR.