Blackmax 200 - Prop issues or something else?

[dsiekman]

Last summer I picked up an old hydra sport 21 center console with the kevlar hull and a late '80s blackmax 200. It is a bit of a frankenstein motor as we intended to repower almost immediately (haven't gotten there yet). I believe the lower is off of a 150 or 175 and is geared accordingly. Last season we were skeptical of the tach readings so I replaced the old regulator/rectifier with a new combo unit. It is currently swinging a 3 blade 23 pitch prop. I think it is stainless if that makes a difference. Anyway, it usually comes out of the hole okay and with 2-3 aboard was running about 30 knots at about 4,000 rpm (based on the tach reading before the rectifier replacement). My first real run of the season was in choppy waters with two adults and four kids. The kids were all forward and I could not get the boat to plane or trim as it should. The spray was coming from very far forward, even at 5500 rpm, and we were only getting about 20 knots. I had written it off to improper loading/driver error/sea state, but is it possible that my prop is spun? I have not had a chance to make it out since other than idle speed which seems fine.

I'm new to outboards and prop sizing so any help is appreciated.

 

[QBhoy]

Wow.
Something is far wrong.
You should defo not be getting 5500 rpm at only 20knts with that set up.
Nor should you be only getting 4000rpm at only 30knots.
Some total experts will be along soon and it's hard to comment without being there or hearing it, but I would suspect your prop Hub integrity firstly and perhaps the validity of instruments second...then perhaps mechanics.
I might not have commented if it hadn't been for the 5500 rpm and 20knts statement. This is defo not right. Does the engine seem as if it's revving this high ? Is there a "slipping clutch" like effect ?
Perhaps may be explained by prop vent holes, but probably not.
Have you checked the compression on the engine too? This may account for the latter statement about 4000 rpm and 30knts, so to may a wrong prop...but the 5500 rpm and 20 knots is a real wonder.
Try and get a video and verify the instruments and data.
All the best.

 

[dsiekman]

Thanks for the reply! I agree, something is wrong in a big way. I took the day off to look into it further and will be headed down to the boat shortly. I'll check the hum first and then move on to the gauge. I'm not used to two strokes other than lawn equipment and dirt bikes back in the day, but the engine seems strong and isn't screaming so I tend to doubt the tach a little. That said, prop hub slipping under load could account for it too.

 

[dsiekman]

It's also possible there are several contributing factors all working together making it hard to diagnose.

 

[dsiekman]

So it's a press-fit hub. How do I visually check to see if it's slipping?

 

[floater212]

mark the prop and hub with paint in a straight line across where hub and prop meet rubber insert, a quick run and check line to see if it moved apart. yes = bad.... no= ???

 

[dsiekman]

So my understanding is that locally having the hub replaced is in the $50-100 range. A new aluminum prop starts around $100-125. In the interest of knowing the prop will not slip do I just buy a new prop? If that fixes the situation I could get the other repaired as a backup.

 

[ajgraz]

Did you confirm that the hub was slipping? If so, if it were me, I'd get the prop re-hubbed, then go from there. Without some baseline speed/rpm data, buying a new prop is just a shot in the dark right now.

If you have determined that the prop hub is NOT slipping, then there may still be some diagnosis to do...

 

[jimmbo]

Last summer I picked up an old hydra sport 21 center console with the kevlar hull and a late '80s blackmax 200. It is a bit of a frankenstein motor as we intended to repower almost immediately (haven't gotten there yet). I believe the lower is off of a 150 or 175 and is geared accordingly. Last season we were skeptical of the tach readings so I replaced the old regulator/rectifier with a new combo unit. It is currently swinging a 3 blade 23 pitch prop. I think it is stainless if that makes a difference. Anyway, it usually comes out of the hole okay and with 2-3 aboard was running about 30 knots at about 4,000 rpm (based on the tach reading before the rectifier replacement).
My first real run of the season was in choppy waters with two adults and four kids. The kids were all forward and I could not get the boat to plane or trim as it should. The spray was coming from very far forward, even at 5500 rpm, and we were only getting about 20 knots.
I had written it off to improper loading/driver error/sea state, but is it possible that my prop is spun? I have not had a chance to make it out since other than idle speed which seems fine.
I'm new to outboards and prop sizing so any help is appreciated.

Without seeing pictures of how the engine is mounted, as in respect to the hull bottom, I'm not going to say the hub is spun. You may be experiencing severe ventilation from a high engine mounting height, heavier than usual load, and choppy water. Some more info would be very helpful

"The kids were all forward and I could not get the boat to plane or trim as it should. The spray was coming from very far forward, even at 5500 rpm, and we were only getting about 20 knots"
That is a bit of a contradiction. Unable to get the boat to plane, well 20 knots is 23 mph. Most boats are planing at that speed.

"Spray coming from very far forward" that could mean it is plowing, as in too much weight up front and/or engine trimmed in too far. However, props usually don't blow out as quickly when engine is trimmed in, unless of course the engine is mounted quite high on the transom

As I said some pics, taken from the side, level with the boat bottom will be worth more than a thousand words

 

[ondarvr]

Also, if the prop starts to spin on the hub it normally gets worse quickly, especially under a heavy load at 5K, it would quickly drop you off plan, and at 5K you may be going 5 mph or less.

 

[Newbie@boats]

Also, if the prop starts to spin on the hub it normally gets worse quickly, especially under a heavy load at 5K, it would quickly drop you off plan, and at 5K you may be going 5 mph or less.

I agree doesn't sound like a spun hub since you are stil going 20knts. My friends boat just spun its hub and it would do 5mph max. Try to goose it and rpms went to the moon and the boat stopped moving.

 

[dsiekman]

Without seeing pictures of how the engine is mounted, as in respect to the hull bottom, I'm not going to say the hub is spun. You may be experiencing severe ventilation from a high engine mounting height, heavier than usual load, and choppy water. Some more info would be very helpful

"The kids were all forward and I could not get the boat to plane or trim as it should. The spray was coming from very far forward, even at 5500 rpm, and we were only getting about 20 knots"
That is a bit of a contradiction. Unable to get the boat to plane, well 20 knots is 23 mph. Most boats are planing at that speed.

"Spray coming from very far forward" that could mean it is plowing, as in too much weight up front and/or engine trimmed in too far. However, props usually don't blow out as quickly when engine is trimmed in, unless of course the engine is mounted quite high on the transom

As I said some pics, taken from the side, level with the boat bottom will be worth more than a thousand words

Thanks, guys, sorry I've been tied up at work and was out of town for a week.

So the cavitation plate on the OB is +/- level with the keel/bottom of the hull. See above.

True, most boats should be planed off by this point. A better way to describe it would be that the boat was running very "flat" or parallel to the water. The spray typically seems to come from about the middle to rear 1/3 of the hull. The spray was coming from well forward of that point as if the bow had never left the water. Unlike other small boats I have been on, this one does not have much bow lift when coming out of the hole. In fact, last fall I lost my trim pump motor when it was trimmed up and running nice. I was still able to plane out quickly. This day in particular, adjusting the trim up or down had zero impact on the way the boat was running.

I know one of the previous owners of this boat (not the one I bought it from) and he claims that even with a smaller OB (Johnson GT 175), the boat ran well and would still do upper 40s to low 50s with a full tank of gas, a full cooler and 4-6 adults on it.

 

[dsiekman]

One question, the transom is wet but seems to be pretty solid. The mechanic (pretty big guy) stood on the cavitation plate and there was no visible movement or flex in the OB. Is it possible that a wet transom was flexing?

 

[Maclin]

Are you sure the trim is working out on the water?

 

[jimmbo]

True, most boats should be planed off by this point. A better way to describe it would be that the boat was running very "flat" or parallel to the water. The spray typically seems to come from about the middle to rear 1/3 of the hull. The spray was coming from well forward of that point as if the bow had never left the water. Unlike other small boats I have been on, this one does not have much bow lift when coming out of the hole. In fact, last fall I lost my trim pump motor when it was trimmed up and running nice. I was still able to plane out quickly. This day in particular, adjusting the trim up or down had zero impact on the way the boat was running.

Ok that does shed more light, As does the picuture. Whoever rigged the boat on the trailer didn't do a very good job. See all that unsupported distance between the back trailer rollers and the transom? Those rollers are supposed to be under the transom. When they are located like on your setup, the weight of the motor can over time reshape the hull creating what's called a Hook. That would keep the bow down.
You also have hydraulic afterplanes(trim tabs), if these are lowered they too will force the bow down

Have you ever weighed the unit? You say you have a wet transom, perhaps you also have wet floor and wet floatation.

 

[dsiekman]

Are you sure the trim is working out on the water?

No, I am not. It works at the dock without issue, but I am not sure it was working under way.

 

[dsiekman]

Ok that does shed more light, As does the picuture. Whoever rigged the boat on the trailer didn't do a very good job. See all that unsupported distance between the back trailer rollers and the transom? Those rollers are supposed to be under the transom. When they are located like on your setup, the weight of the motor can over time reshape the hull creating what's called a Hook. That would keep the bow down.
You also have hydraulic afterplanes(trim tabs), if these are lowered they too will force the bow down

Have you ever weighed the unit? You say you have a wet transom, perhaps you also have wet floor and wet floatation.

Thanks, I will definitely adjust the trailer.

Tabs were up.

No I have not weighed it. And yes, very possible that more of the boat is wet. Next winter I am planning to cut out the deck to do some stringer repair and rerigging and will likely rebuild the transom and (of course) the floor at that point.

 

[jimmbo]

Thanks, I will definitely adjust the trailer.

Tabs were up.

Next winter I am planning to cut out the deck to do some stringer repair and rerigging and will likely rebuild the transom and (of course) the floor at that point.

You suspect broken stringers? Ok, if a hull has weak or no structural reinforcement, all bets are off was to how it will behave when used.

 

[dsiekman]

You suspect broken stringers? Ok, if a hull has weak or no structural reinforcement, all bets are off was to how it will behave when used.

Not exactly. Where the bottom of the stringers are notched to allow water to drain to the bilge, the wood has deteriorated. This notch is right below where the stringers tab into the transom. Based on the shape of the glass at the notch it appears as though the notch was original to the boat, not a hack job by a prior owner. That said, the adjacent glass is pretty thick and there is no sign of cracking or damage. My understanding (admit it is limited) is that the wood within the stringer is not always intended to be structural, but is rather sometimes just used as a mold for the glass. There are not signs of stress cracks in the hull or deck and no soft spots that would lead me to believe the stringers are failing or have failed. That said, I have not found any information as to whether this hull was designed with the wood being integral to the structure or not. I prefer to play it safe and over-engineer whenever possible so they will be repaired.

 

[dsiekman]

Leaning towards wrong tach setting.

Went out tonight. Boat came up on plane and ran fine. Trim was fine. Still getting about 30 knots at 5500. Just for grins I pushed the throttle all the way forward and topped out just shy of 7000. Backed off quickly but it sounded happy if that makes sense.

I do not know how long these gauges have been on the boat but do know it had a Johnson GT on it at one point. Maybe a different setting?