New Prop or Trim Tabs?

[dorchard]

Hi Everyone,

I have a 1991 21' Mariah 2000Z open bow runabout with a mercruiser 7.4/bravo 1 (300HP), and I'm not satisfied with the holeshot/overall performance.

Recently i've been doing more and more wakeboarding, and i waste all of my energy holding on while the boat plows water with the bow in the air and lugs onto plane. I thought that a boat this size w/ this much power shouldn't have this problem...but nonetheless...

The dry weight of this boat is around 2800 pounds, and we usually have about 6 adults on board. We also have a couple hundred pounds of pea gravel under our bow seats to try and keep the bow down.

I'm currently running a Quicksilver Mirage 21P (14 3/4" Diameter??) prop. At WOT I'm reaching about 4150RPM, and get to about 55MPH with the outdrive trimmed as far out as it can go without losing grip. The hull has a 20degree deadrise.

I'm trying to figure out if i should go towards a lower pitch prop with more blades (4 or 5), or a set of trim tabs first. I live in central illinois, so there are no boat dealers around that I can find that will let me demo a prop (a couple of places laughed at me for asking...needless to say i won't be doing business with them!).

I don't have a large budget to work with, so I'd prefer to only buy what i need, and it would be great if whatever solution I choose will help fuel economy (with gas prices already rising!!).

Any insight you have would be greatly appreciated!

Thanks,

Dan

 

[hwsiii]

Re: New Prop or Trim Tabs?

dorchard, what is the gear ratio for your motor.

H

 

[Varago]

Re: New Prop or Trim Tabs?

I think you would find that Smart Tabs would do 4 times the job any prop change would do for 1/3 the money. I did the prop change first went with a 21" 5 blade high five from the 21" 3 blade BalckMax both would turn 5k. Then when that was not enough I picked up Smart Tabs and WOW night and day.

Dave

 

[mrdude]

Re: New Prop or Trim Tabs?

ditto!!!! you cant beat the smart tabs... like he said.. night and day difference

 

[nlain]

Re: New Prop or Trim Tabs?

Dorchard, What is the wot rpm of your engine supposed to be, I would think it should be somewhere between 4600 to 5000, you are only turning 4150 at wot and 55 mph, good speed but the wot rpm is low. You first need to prop the boat so engine will reach full wot, also make sure engine is running properly and developing full torque and hp, then if not happy look at the Smart Tabs. I use Smart Tabs and will say they do everything advertised but to get full benefit you need to have the boat, engine and drive performing properly before you install tabs.

 

[dorchard]

Re: New Prop or Trim Tabs?

Hi All,

Thanks for the replies thus far! As per my clymer manual the gear ratio is 1.5:1, and WOT should produce 4200-4600RPM.

I was thinking of an 18P or 19P 4 or 5 blade prop (if I didn't do the trim tab option) to put my WOT in the higher end of the range, and hopefully boost the holeshot. I know that 55MPH isn't bad for a boat this size, but I've been a bit disheartened hearing people get faster speeds with smaller engines on bigger boats...it just seems like i'm missing out somewhere.

As for tabs, the concern i have is that as i'm doing wakeboarding, i've heard that tabs can make the wake smaller, which is not what i want...and as the smart tabs aren't adjustable (if i understand them right), what i get is what i get. So i was thinking the bennett ones i could trim in to get on plane and then trim back out to keep the wake nice and big...also i was wondering how much benefit i might get from adjustable tabs through various running conditions, like if i go somewhere with rougher water.

Any more/different ideas?

Thanks again,

Dan

 

[wca_tim]

Re: New Prop or Trim Tabs?

whatever you decide about prop pitch and tabs, get a vented prop - it works a lot like a stall converter on a street car letting the prop slip more out of the hole and getting rpms up where it makes more power faster. the merc props with adjustable vents (via plastic inserts) are great.

I also think you would be dissatisfied with a 5-bladed prop. everyone I know that has bought one has been dissappointed with the top end.

In addition, that big block is a fair bit of weight in the azz end of the boat and would probably ride better with a neutral / stern lifting 4-blade than a mirage. A mirage has a lot of bow lifting character and only makes the bow rise issue worse.

something like a mercury bravo, or if you run across one a hydromotive quad 4 in the right pitch range on ebay or something, they're GREAT props. both are a good bit faster than a mirage on my boat.

I think you would be ecstatic with a cheap set of smarttabs and a vented 4-blade stern-lifting / neutral prop in 19 or 20 pitch. (i might shy away from the plastic version of smartabs, i broke a pair off skipping across the waves...).

if you can't find what you want in a vented set-up, a mach 4 sterndriver is good (used of course) and the solas titan 4 blade (not the HR / "high rake" version - it's for bow lift that you don't want).

note that with the wakeboarding... you can adjust smarttabs by moving the bolt, etc... and I've found that I can get just about as big of a wake by trimming out (driving the stern into the water) but can also maintain a plain much more easily at lower speeds(ie 13 or 14 pmh...).

ps. Where are you on the water the most?

 

[dorchard]

Re: New Prop or Trim Tabs?

Hi Tim,

Thanks for the advice, i'll start pricing out your recommendations!

We mainly boat at Clinton Lake in central Illinois (nuke lake, so its always nice and warm!) Last year we did a trip to Kentucky Lake, and we would like to have the option open to go to places like Lake of the Ozarks and not have our boat setup incorrectly, hence the questions about bigger waves, etc.

Thanks again for your advice!

--Dan

 

[wca_tim]

Re: New Prop or Trim Tabs?

on budget and with the water you're in you might go with a set of the nauticus sx series smarttabs (yes plastic... they should hold up and if not the company will likely replace ehtm with the steel ones..., the sx series worked better than the steel ones before they broke, note that I'm probably running 20 mph faster than your set up and they broke skipping across waves in the sound...), and see how that does before putting the 300 - 700 into a new prop... Is your mirage one of the newer ones that has the vent holes drilled in it? Most came with black plastic plugs in the holes. Assuming that you have through prop exhuast, You can get inserts that have different hole sizes in them, the bigger the hole the more exhsuat gas it expels along the leading edge of the prop blades when you take off. when going faster they don't make any real difference, but can do wonders for getting your rpms up on take off and cause a dramatic difference in hole shot...

 

[dorchard]

Re: New Prop or Trim Tabs?

I went ahead and bought the stainless version of the smart tabs, because the top of the actuator needs to mount to the underside of my swim platform (and in this scenario the manufacturer recommended the stainless version).

I should have them the middle of next week, and put them on for boating in the first weekend of June. I'll report back my findings!

Thanks again for your help!

--Dan

 

[Gryphon]

Re: New Prop or Trim Tabs?

I was in a similar situation, (but a smaller boat/engine). I was told to install the PR500 which inlcude an adjustable lever which allows the tabs to be up or down depending on use, i.e. Wakeboarding or not. Found them for $26 from online marine I believe. Must install them when installing your tabs.

 

[wca_tim]

Re: New Prop or Trim Tabs?

I went ahead and bought the stainless version of the smart tabs, because the top of the actuator needs to mount to the underside of my swim platform (and in this scenario the manufacturer recommended the stainless version).

I should have them the middle of next week, and put them on for boating in the first weekend of June. I'll report back my findings!

Thanks again for your help!

--Dan

is your current prop a mirage plus drilled with vent holes? Often plugged with black plastic inserts. If so, you can have a major impact on your holeshot by changing the inserts (we're talking a few dollars here)... if you have through prop exhaust.

btw, I grew up not 4o miles from you...

 

[dorchard]

Re: New Prop or Trim Tabs?

I've attached a pic of my prop...where would the vent holes be?

And where abouts was it you grew up? I live in Urbana now, but lived for a couple years in a little town called Cerro Gordo (near Decatur) before that.

--Dan

 

[mrdude]

Re: New Prop or Trim Tabs?

btw... the smart tabs are adjustable, u have probably found this out already thou..

 

[braddog777]

Re: New Prop or Trim Tabs?

Fascinating thread. . . I'm having a similar problem, the nose of my '83 SRV 197 (Merc 260) has a tendency to bounce regardless of how it is trimmed. I just bought it recently. It has a repaired stainless 19. . . never new a prop could control attitude/angle.
It has very low hrs and runs strong. I grew up with an SRV190 (Ford 302) which seemed to be much more stable in the water.
It's closed bow must have added weight to the fnt.

This is freak'n out my wife. the prop repair is marginal and needs to go. . . oh BTW, I picked up some Vortec heads with mild port work done and will bold on a Victor Jr. intake which is slightly weaker at the bottom end. I'm interested in this vented prop technology to compensate.
this will push close to 310 to 320 horses when done. i may consider a 21 with this Vented technology.

Am I on coarse?

 

[dorchard]

Re: New Prop or Trim Tabs?

Hi All,

Sorry for the delay in reply...we had a nasty little issue our first run with the new tabs...electrical short, lots of smoke, and a (friendly, luckily not professional $$$$) tow back to the marina. Well I sorted that out, and finally got to take the boat out with the new tabs, and man, if I had these things on there from the beginning, I don't think i would have ever given much thought to "getting on plane". The boat just steps right up on to plane effortlessly, and at about 4-5mph slower than before, too!

One issue that I am having, however, is that the boat is listing hard to port. I'm pretty sure i installed the tabs even on both sides, though i might go to harbor freight and buy a tool to measure the exact angle, and the boat never did this much before. The weight balance in the boat has never been that much of a problem...we do have a built-in cooler only on the port side of the boat (which was full), but that has never caused too big an issue. My wife and I had to both sit almost on the same side of the boat to make her run level. I hate to ask passengers to move around to excess, and it seemed like the weight balance was really tricky.

Any ideas? I can take a picture of my installation if desired...the gas shocks did have a bit of a funny mounting position due to the angle of my swim deck.

Thanks!

Dan

 

[junior1113]

Re: New Prop or Trim Tabs?

i would look for a 18 pitch 4 blade. and run that at about 4500 and get 300 hp. at 4150 you are getting like 270hp hp. i would tabs on any boat mutch better ride. 4 blades cruse on plane at lower speeds great for wake boarding.

 

[nlain]

Re: New Prop or Trim Tabs?

I would say to double check the adjustment, you may have more pressure on one side, you have both height/angle and pressure adjustments, unless you got the protroller top brackets they do not adjust. You are getting the bow out of the water when you trim up aren't you, if not then you need to adjust to a lighter pressure setting and check how it runs then. You may have to play with adjustments to get the balance right but I feel sure you can.