Mercruiser Prop Drive frustration...please help

[othelloears]

99% done witha complete resto of a 1984 Welcraft Elite 200 bowrider and have done many posts.

It has a mercruiser 228 (305 chevy 4bbl 228 HP) with a 1.5 Alpha drive.

My main question is should this have a 1.5 or 1.62 drive on it? and more importantly is there any way to confirm 100%.

When I bought it the seller put a new 21P prop on it and it wouldnt plane at all and would only turn about 4000 rmps WOT.

I put a 17P on it and it planes out great but still only turns 4400...which is minimum WOT. It also has difficulty staying on plane. Basically can't do less than 2800 rmps or lose it cruising. Seems high to me.

I tried a 19P this weekend and could only get @4200 rpm WOT

I have to believe this boat should be able to handle a 21P if not a 23P never mind a 19P. I have ruled all possible problems out...Boat is completely restored...no wet foam etc... Engine is tuned and strong with good compression to all cylinders.

My thought is that the only thing this could be is the wrong drive ratio. Based on my rough math that would account for the missing WOT RPMs

Its got a 1.5. Most people I talk to say that is probably right but I have read in other places the small v-8's could run 1.62.

Can anyone out there tell me for sure what drive should be on this engine or direct me to where I can find out.

Thanks

 

[rodbolt]

Re: Mercruiser Prop Drive frustration...please help

most 4.3 and 5.0 used the 1.62.
I have seen the 1.98,1.62 and 1.5 used.
makes some engine efficiency issues but propping can get the RPM back in range its just not as efficient.

 

[othelloears]

Re: Mercruiser Prop Drive frustration...please help

Thanks its great to have finally have some confirmation of what I suspected.

Little confused by your comment though on efficiency which is obviously what I am frusterated with.

I assume your comment to mean I likely should be running a 1.62 drive which would more efficient but its ok to run the 1.5 with the smaller prop just not as effiecient. Is that what your saying?

Thanks

 

[liquidlew]

Re: Mercruiser Prop Drive frustration...please help

Sounds like you figured it out yourself. You have to keep going smaller on the prop pitch to get the correct rpm, so it seems the 1.5 is too steep. I have a bud who has a 20' Mariah with a 5.7 and his ratio is 1.62 and runs a 21p and it's right on the money on the rpms. Seems to me you at least have to go to the 1.62, or maybe even the next one up? Funny, I just had a ratio issue, I have a 4.3 V6, and had a 1.50 ratio on back (newbie, didn't know any better), and could never get over 4000 rpm, even with a 17p and no one in the boat. I am supposed to have a 1.84 ratio, so I got one and put it on and what a difference! Now it's 4600 rpm with a 21p. Previous owners do some strange things!

 

[Alpheus]

Re: Mercruiser Prop Drive frustration...please help

My SeaRay came stock with a 1.5 ratio and a 350. Im running a 19 pitch and hitting 4600 rpm at WOT...

How old is your 305???

 

[othelloears]

Re: Mercruiser Prop Drive frustration...please help

I believe its original...1984 with 900 hours. Compression seems fine though believe it or not. The resto cost 10x what I planned so can't justify engine now.

I assume I will be repowering in the next few years. I can't justify new risers no this engine so just waiting.

The alpha is brand new. Thats what pissed me off more. I think the guy threw on a junk 1.5 to sell it and stupuid me bought a replacement 1.5 so I am stuck with the wrong drive.

My long term plan is to go to the 350/5.7/260hp which are actually cheaper (more common) when I repower which will better match the 1.5 drive.

 

[chaparall villain 2]

Re: Mercruiser Prop Drive frustration...please help

i had a 350 and mine was 1.5 ratio running 23 pitch mirage and turns 4600 wot and its an alpha outdrive ... some would depend on the weight of the boat too

 

[othelloears]

Re: Mercruiser Prop Drive frustration...please help

Now that it appears clear I have the wrong drive which I won't be changing anytime soon wanted to get some opinions as to make the best of the situation.

As I said can't run the 14 3/4 x 19P 3 blade Merc alum prop I have on there but would like to improve from the 14 x 1/2 17P 3 blade merc alum which gives me good thrust but trouble holding plane and less top speed.

I don't have GPS yet so no speed calc

Options I am aware of are (1) bigger blades (15-15 1/2 x 17P) ;(2) 4 blade; stainless; 18P; better prop than the mercs with less slip.

What are your guys thoughts?

 

[captk]

Re: Mercruiser Prop Drive frustration...please help

Some 898s came from the factory with 1.5 gears. try a 15P prop.

 

[stonyloam]

Re: Mercruiser Prop Drive frustration...please help

Information here: http://www.seilermarine.com/info/mercruiser-drive-ratio-chart.htm

 

[stonyloam]

Re: Mercruiser Prop Drive frustration...please help

When I bought it the seller put a new 21P prop on it and it wouldnt plane at all and would only turn about 4000 rmps WOT.

I put a 17P on it and it planes out great but still only turns 4400...which is minimum WOT. It also has difficulty staying on plane. Basically can't do less than 2800 rmps or lose it cruising. Seems high to me.

Turning 4K rpm and not being able to plane sounds like a trim problem rather than a prop or outdrive problem. If you get up on plane, you should pick up at least a few hundred RPM, and be right in the ballpark for WOT especially with that size boat. You might look into trim tabs, either passive or power (a friend of mine just added power tabs to his boat and it made a huge difference). Good luck.

edit: After reading your other posts, I suspect that during the restoration you may have changed the hull shape slightly, eg. removed some of the "hook", and that is the root of your current "difficult to plane" problem, Just an uninformed guess.

 

[othelloears]

Re: Mercruiser Prop Drive frustration...please help

I think I confused you guys. With the 17p it planes quickly and nicely and have 4500 rpm @wot which is within the range. What I don't like is I lose plane when cruising at about 2,600 rpm. When I make long runs to the shoal I try to cruise around 2750...Isn't that common?

I actually used the chart posted which is why I bought the 1.5. I understand that was only guildlines and changed based on boat size weight etc...

For you math experts out there I assume if I go from a 1.47 drive to a 1.62 drive I should gain about 10% in RPM (1.62/1.47) or 440 on current WOT of 4400. Am I in the balpark? That also leads me to the conlusion I should be running a 1.62 because that would put me in a 21P which was originally on the boat. Sound Right?

What is the downside to going to the 1.62...am I going to tear up the gears?

 

[stonyloam]

Re: Mercruiser Prop Drive frustration...please help

For you math experts out there I assume if I go from a 1.47 drive to a 1.62 drive I should gain about 10% in RPM (1.62/1.47) or 440 on current WOT of 4400. Am I in the balpark? That also leads me to the conlusion I should be running a 1.62 because that would put me in a 21P which was originally on the boat. Sound Right?

The engine RPM would increase, but wouldn't the prop RPM remain the same? I ran the numbers on both, and the prop RPM would be about 2700. so at 4400/1.62 or 4000/1.47, I don't see where the 21p prop would work any better. And you are still going to fall off plane at the same hull speed and at a higher RPM. I am still convinced that a good set of trim tabs is where you should spend your money. Don't mean to be argumentative, but that is just the way it looks to me.

 

[othelloears]

Re: Mercruiser Prop Drive frustration...please help

I apreciate your input and don't think your argumetative. I am no expert but I been boating for 30 plus years and do disagree.

First, I think trim tabs help in most boats but do not think its my problem. I get up and stay on plane at what I would consider normal speed and rpms with the 17 pitch prop. Based on all the info I have it is definately a gear ratio issue.

With the raio I have I am forced to run a 17 to have WOT rpms. I tried a 19 this weekend and it bogs.

From a pure efficiency standpoint the size and weght of the boat needs a more agressive pitch than a 17 which I have to run on this ratio. Technically I believe I have a ton of slip trying to push the heavy boat with a flatter prop. I cannot run the numbers becuase I don't have GPS (yet) but the boat will run much more efficiently on a 19, 21, or even 23 pitch.

I hope to have the set up for next season and have the numbers to back up my theory.

Again thanks for the input

 

[chaparall villain 2]

Re: Mercruiser Prop Drive frustration...please help

i have a 20 ft chaparall villain and i have always had problems loosing plane ... on mine i put a hydrofoil on the outdrive cavitation plate for v8s .. i didnt loose any speed i can see and it stays on plane at lower rpms .. the props you running aluminum or stainless?

 

[othelloears]

Re: Mercruiser Prop Drive frustration...please help

Right now the only prop that runs acceptably is a 17P merc alum. Great hole shot but weak on the top end and ability to hold plane. Although I have to say despite boating for 30 years I am amazed at how hard you guys run these boats. I try to cruise around 2900 rpm and was complaining about losing it at 2650. It sounds like from reading this site that is better than normal. Consensus cruising rpms in here looks like 3500-3700...That seems aweful hard.

Based on all the info I decided to stick with a 17P but add diameter and SS. I found a used SS Mirage Plus 17P which is oversized (15 1/2 vs the 14 1/2 now). I hope to get it this weekend and will report back

As I said I expect to repower in the next few years and will go with the 275HP 350 vs the 228HP 305 I run now. That will fit my 1.5 unit better and I would expect to throw a 21 or 23P easily

Thanks for all the input.

 

[othelloears]

Re: Mercruiser Prop Drive frustration...please help

Just wanted to let you guys know that I was finally able to get the new prop on. I ended up getting a great deal on a used Mirage Plus stainless. I thought I'd go with the 17p but liked the idea of the oversized prop. All I can say is WOW! What a difference. Its funning becuase the first time I hit the gas the wife actually noticed the difference. I haven't been able to check the rpms becuase of the seas up here since the hurricane but the performance is remarkable. I am looking to doing some more refining once I get the gps and have speed readings.

Thanks for all the help